The first is that it is built from the ground up around a flat social network and that maps really poorly onto real social networks. Sure they can add things like groups to try and address this but it's baked into the DNA of the site and the way people think about the site. The blog post that accompanied the recent profile refresh really brought home how they do not understand this. Listing work projects and who you worked on them with on a friend-oriented service? Promoting parts of your life so more 'friends' can learn about them? Who on earth wants to do this stuff?
The second, which is part of the reason for the first (college social networks are relatively speaking flat) is that Facebook was built and initially populated by the Ivy League. It was the classy, exclusive social network. Now it's everybody's, there's like buttons on every site, every brand from downmarket crap to luxury. Every market has price differentiation, social networks will be no different, there is a huge opportunity for a more exclusive, more upmarket network and when those users bail on Facebook there's a real chance it'll cause a cascade.