For the first half of a year that Facebook Platform was available, you could only build applications that ran inside the Facebook site itself. On December 12, 2007, Bebo launched its new application platform, which was carefully designed to use the same architecture and virtually identical tags as Facebook Platform (Bebo collaborated with Facebook on the development). Shortly thereafter, Facebook announced that it would be opening up the architecture and making it available to other social networks as a model for building out application platforms, and would even go so far as to license its technology to interested parties (see http://developers.facebook.com/specification.php for the high-level specification). In a world of things that are too new and young to predict, this is a particularly fresh area and it’s difficult to anticipate the effects and consequences. The potential promise is that your Facebook application might run unaltered (or basically unaltered) on a variety of other websites in the near (or somewhat near) future, which really just sweetens the Platform pot, but don’t count the proverbial chickens until they hatch.
Facebook Connect, launched in May 2008, takes the idea of Facebook Platform off-Facebook one step further by enabling you to include Facebook-like features in your own site. The topic is a little outside the reach of this book, but you can find more information on the Developers Wiki at http://wiki.developers.facebook.com/index.php/Facebook_Connect.