The fact that a phone's contacts are accessible to other apps is the way that most of these bootstrap.
The social graph is ok for some types of networks (messaging), and was ok back in the early days of social networking, when most people were starting from almost zero, but at this point, if you just ported the social graph but didn't bring over all the content (personal photos, videos, posts, etc) I think a lot of people wouldn't find that enough. Even if you decide to make a move, it's hard to break the habits of everyone else in your graph and have them post their stuff and message you somewhere else too. The land grab part of this might be over.