I doubt it's over. Facebook has their constellation of of apps and services. And the cost of switching is not so simple for, say, the average user who's used facebook as their photo and video archive over the years. But most of all they have the opportunity to pivot, reinvent themselves to keep relevant. Something I wouldn't count them out of. Personally, I hope they fail. The last two years have seen my feeds turn into majority hate filled posts full of vitriol for some issue or other. Only a minority seem to actually be relevant for keeping in touch with friends and loved ones.
They are going to fade into irrelevance for the same reason nightclubs do, they just aren't cool anymore. They know this which is why they are on a mission to buy, kill or copy every new social service that gets traction.
Under that logic (which I don't dispute-- it might be true) all such platforms are ultimately doomed to irrelevance and failure. Some even start out that way (Google+) But I do wonder if there's a model out there that could have longevity. If so, I have to think it's success would in part be predicated on not stuffing every other interaction with ads and paid content. Facebook's mission is to "bring the world closer together", but they really don't do that. Even at their best they've only been a showcase for trite, shallow interactions. That may be an over generalization, there's probably some few who find more value there, but I've seen it cause more conflict and social rifts than getting people closer together.
I think the model that "could have longevity" is the web, which has longevity precisely because it is a wild west of different viewpoints and competition.
It's the same old cathedral vs bazaar or state owned corps vs free market situation again.