Facebook is going to forge a new path to irrelevance, not take the same path as MySpace. People will stop using Facebook because (1) they outgrow it, (2) they get tired of it, or (3) it lost its usefulness.
I don't believe another app will take its place. Rather, I think an existing app/platform is already Facebook's biggest competitor. And it's one you've been using for 30 years. Email.