People make way too much of the Facebook platform. The part that's relevant to innovation is allowing third-party apps access to a list of who and what a user likes. It's silly that people are obsessed with building those apps inside Facebook, though. Look at use rates for the various apps. Facebook could get rid of every app except the basic profile, Photos, messages, the Wall, and Events, and it would be just as popular. The rest isn't nearly as promising.
One of the main differences between the facebook platform and the windows platform is that on the web, it costs very little cash for users to leave for something new. When desktop hardware and OS installations were involved, people stuck with their first choice (which was in large part determined by what was available in local retail stores).<p>It looks like people do the same thing for social networks, because they do invest time into their profiles and connections and such; but it's still far less of an investment than someone typically would have made in a PC back when Microsoft became the dominant desktop platform. It takes all of 5 minutes and zero dollars to switch from Facebook to Myspace, or Orkut, or whatever. Even if you had a desktop that could run all of the other competing desktop OS platforms(which wasn't possible for most people) it took a lot of work to switch between them. If you were already using windows, you didn't even think about using anything else.