I'll be honest, the first comment that said Facebook would be the next to leave caused me a big surprise, but the logic behind the reasoning was there.
That said, of all the social media networks that exist (excluding perhaps Google+) they have one of the largest teams and remain deeply and intimately connected to their network; what is popular (ie, trends) versus what it is not, what people want, how they can make the FB more attractive. The fact is that that is their identity and they are very good at that, which is part of the reason why Myspace followed the path of the dodo. Not only that, but it is also beginning to incorporate professional aspects in the groups and the advertising space. While you can do business, you will be there. Therefore, the objective of taking decades is probably the most accurate of the lot with respect to the eventual disappearance of Facebook (in my opinion anyway).
Instagram has become more than just sharing photos, relying on the realization of Facebook that while images say a lot, people still want to connect on a more personal level. Even if it's just a facade through a mobile device or computer screen.
Google+ will probably be available for a while for the simple fact that it is funded by google.
Twitter, unfortunately, does not seem to go anywhere and, if you ask me, is directly responsible for the slow death of spelling, grammar and attractive writing.
I can not see LinkedIn going anywhere, if something is becoming more popular.
So really, that just leaves Snapchat, that when you compare it with other social networks it really is not one. It just does not compare in terms of ways people can connect, share experiences and find news.
Only my two cents!