I don't think Facebook will die, for two good reasons: 1) there are many more people on facebook than networks like myspace ever had. This makes a transition much, much harder. 2) facebook is not only used for facebook, but for many third-party apps as well. People will keep their facebook accounts so that they can log in to external services.
Re: Social Networks Implode Quickly
#21Regarding 2), I'd like to point out that just because you "leave" Facebook for another social network doesn't mean that you'd delete your Facebook account. In other words, the decision to use a new social network wouldn't preclude you from still logging in to other services with your FB login.