Veterans of the software industry have, hardcoded into their DNA, the assumption that in any fight between a platform and an application, the platform will always win.
what Facebook is now doing is a lot more sophisticated than simply MySpace-style embedding: Facebook is providing a full suite of APIs -- including a network protocol, a database query language, and a text markup language -- that allow third party applications to integrate tightly with the Facebook user experience and database of user and activity information.
And then, on top of that, Facebook is providing a highly viral distribution engine for applications that plug into its platform. As a user, you get notified when your friends start using an application; you can then start using that same application with one click. At which point, all of your friends become aware that you have started using that application, and the cycle continues. The result is that a successful application on Facebook can grow to a million users or more within a couple of weeks of creation.
Finally, Facebook is promising economic freedom -- third-party applications can run ads and sell goods and services to their hearts' content.
Meanwhile, some people seem to be doing well on it.