News feed - The news feed is the most powerful component of Facebook. Period.
There are two ways that applications can leverage the news feed. The first is naturally built in. Most of the time, when a user adds an application it is displayed in their friends’ news feeds. While it is not a guarantee that it will show up in other people’s news feeds (due to a number of factors pertaining to news feed optimization), this is the primary thing that helps applications spread virally.
When I launched my Bush Countdown clock I added no viral components and relied completely on people’s news feeds. This has spread the application to close to 8,500 users. While not spectacular, it is hardly something to sneeze at.
totally agree with Nick's first sentence above. as a followup to my previous Marketing Facebook Apps post & Justin Smith's News Feed Optimization (NFO) post, i'm going to try to do more posting in the near future on Facebook News Feed & mini-feed... as soon as i figure out some more interesting shit to say.
(ps - don't hold your breath.)