[note: this post was originally a comment on TechCrunch post summarizing Max Levchin's panel on web platforms from the Web 2.0 Summit, with Facebook, MySpace, Google, & Microsoft. i decided since my comment went past 3 paragraphs, i should probably expand it on my own blog...]
this is gross over-generalization/simplification, but platforms are successful if/when they have:
1) code libraries / feature abstraction -- so developers don't have to build everything
2) customer distribution -- so developers don't have to market everything
3) monetization -- so developers don't have to implement/collect payment
it's arguable there are many other platform issues & requirements -- security, scalability, standards, metrics, user authentication & support, etc. however these 3 primary aspects are what make platforms tick. sometimes platforms can be successful if only 1 or 2 of the items above are working, altho if you want a dominant platform it's helpful to have all 3.

Here are my incredibly unscientific and from-the-gut "Platform Bookmaker Odds" on Dominant Future Platforms:
Facebook (3:1) has arguably been the platform leader in the past year or so, at least in terms of innovation & inspiration. They've done an excellent job with the first 2 points, but largely failed in terms of monetization to date. And because their distribution incentives were creating a negative user experience backlash, they decided to dial back distribution & revise User Profiles in ways that dramatically limited the # of platform app winners (but there are still winners). Also, until FB has payments in place, and has setup aligned monetization with their developer community, they are likely to be reluctant to really open the floodgates on distribution. If/when they have payments rolled out, expect them to spin up distribution again, probably through 3rd-party news feed stories. And people still underestimate how much power they can bring to bear if Facebook Connect starts working. People were too hyped up on Facebook when they launched (maybe me too), but now they're too pessimistic. Facebook will start kicking ass once they get Connect & Payments figured out... tho might be mid-2009 before that starts happening. But the DNA of this company is all about Platforms & Developers... i'd be very surprised if they don't do very well over the long-run.
Microsoft (5:1) altho the platform Godfather historically, is far from having a functional web platform, primarily because they have not figured out a web distribution story for 3rd parties (nor a monetization story either, tho that's fixable quickly via acquisition or partnership). while they have tremendous reach via Windows and the Web, they have not translated that into consumer internet platform distribution opportunities. they also do not have a payments story (good reason to buy eBay/PayPal or Amazon, but don't hold your breath). Still, given Microsoft's historical dominance & belief in platforms & developers, i'd expect them to get there eventually... but it may take years. expect them to be a player, but i'd guess it may take a lot of acquisitions & luck for them to maintain the app dominance they had in the 90's. They'll be around, but they're not leading the way right now. (update: Ballmer sez MSFT is planning to copy Apple's app store... not a bad idea. again, like the GOOG, MSFT can KILL if they simply enable 3rd-party access to their amazing distribution & reach).
Google (2:1) certainly has the cojones & brainpower to do platforms, and they kick ass on features, but up until last year they were pretty piss-poor at developer evangelism. However, you could argue Google's biggest "platform" is really just search (& indirectly SEO/SEM, and also analytics). while they are a juggernaut in search, and their potential distribution is phenomenal, their initial platform efforts left a good bit to be desired. But like Microsoft, Google certainly understands the benefits of a developer / platform strategy and there is good reason to be optimistic for the future. OpenSocial has a long way to go, AppEngine is still fairly new, but Google is now headed in the right direction... and if Google ever *does* open up properties like YouTube and Gmail and the main search platform to developers, the sky's the limit. Google has no lack of distribution, but again like MSFT it's mostly direct, not on behalf of 3rd-party developers. Google also has Checkout for monetization when they get their shit together. Expect them to put a lot of horsepower behind catching up to Facebook's surprising early platform leadership, and steamrolling ahead to a dominant position. They're playing a bit of catchup, but they are certainly the odds-on favorite horse to win the long-term race.
MySpace (7:1) does seem to be following cautiously in Facebook's footsteps, and given their existing footprint in music & entertainment (not to mention their already significant spam issues with user communication), it seems appropriate for them to move slowly. But they have shown progress over time, and if/when they implement payments they will likely attract a good deal of developer attention (and in fact, since FB dialed down distribution, MySpace has already drawn some of that audience in). While i expect MySpace to primarily focus on their successful entertainment verticals in Music, Movies, Television, and perhaps an emerging presence in Games, they will likely do quite well due to their excellent distribution & monetization opportunities in those areas. While everyone thinks Facebook is pulling ahead of MySpace, in fact MySpace is a much safer vertical play in entertainment. They may only be successful in selected areas, but they will likely do quite well there.
not present on Max's panel / not mentioned in TC article or my analysis above: Amazon (2:1), Apple (3:1), SalesForce (4:1), YouTube (4:1), Yahoo (4:1), Mozilla (7:1), LinkedIn (10:1), Wordpress / Automattic (10:1), Twitter (10:1), FriendFeed (12:1), multiple other blogging platforms / social networks (15:1) Typepad, Blogger, Hi5, Bebo, Friendster, Orkut, other international players. I'm sure i'm forgetting a lot here, but this piece wasn't meant to be completely comprehensive.
A few brief thumbnails on the other main contenders:
Amazon is the defacto platform leader in 3rd-party infrastructure. They have yet to tie this back into the consumer Amazon platform in interesting ways -- largely AWS is completely disparate from Amazon.com -- but if they ever do, then watch out. Amazon has been the quiet leader in personalization & e-commerce for the past 10 years. If they decide to platform-ize that expertise, they will absolutely continue to be a very valuable company. Only question is -- why in the hell haven't they been bought out by Google or Microsoft ?!? They'd be SO much more valuable in either of those 2 larger players empires. Answer? Jeff Bezos isn't done having fun yet. If he ever tires of that job, and decides to put the company up on the block, expect the mother of all battles by MSFT & GOOG to acquire that jewel.
Apple is kicking ass, but remains to be seen if they can parlay initial success in music & mobile into larger app platform success. They don't have a terrific platform history, but the last 5-7 years they've done a great job on distribution, and recently on features & monetization. Assuming they don't decide to screw the developer community into oblivion at some point -- always possible with a mercenary like Jobs -- then Apple will likely remain a force in consumer communication & entertainment platforms.
I'm not an expert on SalesForce, but they seem to be the leading player for hosted business platforms. I'll refrain from much analysis since i don't follow them closely. Yahoo continues to choke on both feet planted firmly next to its own tonsils, but they still do some great work on platforms & developer communities. If only they can show a little business leadership & stability, they'll be around. YouTube is a surprise here, and i list them separate from Google because they really are such a separately dominant player in video. And like Facebook, the DNA of YouTube is all about developers. They haven't really needed to do much since they are so far ahead of everyone else (in video), but i'm bullish they will figure out the monetization and as they do make it available to all of their developer community. Mozilla has platform potential, if only due to an amazing ability to grab browser market share from MSFT. LinkedIn has been a relatively plodding platform, but due to their audience specialization & demographics, they will be highly attractive to developers if they ever decide to open up more significantly (& revenue potential there is off the charts). Of all the blogging platforms, Wordpress / Automattic is the most exciting & interesting... more to be revealed there, particularly if they continue on their recent micro-acquisition strategy. Twitter is rocking these days, and i expect Ev & Biz & team to roll out some cool platforms stuff in 2009. FriendFeed has so much potential it's not even funny, but i'd be surprised if they don't get swallowed back up by the GOOG before long. Several other social networks have also embraced platform strategies, and may have modest success within selected geographies & demographics.
my apologies for any significant oversights or inaccuracies on the above. and the "odds-on" stuff is really more for entertainment than serious discussion -- most all of these companies will be successful to some extent, tho they have widely-varying attention to platform issues. i'll try to make a few updates to this piece as my more-informed readers upbraid & correct me in the comments.
for more discussion of Platform dynamics, see Max Levchin's excellent masterpiece here (max: if you're reading this, please write more often!) : Developer Incentives in Social Networking Platforms
Andrew Chen also has many terrific articles on platform dynamics
too numerous too count... i simply recommend you read as many articles
on his blog as you can: http://andrewchenblog.com
for praise / criticism of Facebook platform, and a few others on business models for social networks, integrating email & e-commerce platforms with social networking, etc see my previous posts here: