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.
It may be different for different cohorts, but for my social circle, SMS is the de facto communication medium. I deleted my Facebook account over a year ago and still keep up with friends, plan events, etc., all via text message.
Thus, there really was no downside in terms of being "out of the loop", while I get the benefit of not feeling like shit because I'm continually inundated with self-promotion posts that make others (most of whom I barely know) look like they are doing more important things than me.
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.