It'll either radically change to keep up revenue and alienate it's audience or it'll go the way of MySpace and Geocities and be surpassed by the next latest and greatest. I say about 10 or 15 more years (at most) and we'll have a large scale walking back of social media. We're already seeing it being less and less important to people.
That i don't believe.. people, especially those in their 20s and teenagers are just switching services to other sites like Twitter, Instagram and whatever else there is.
None of these may reach the humongous user numbers of Facebook but FB has become an everyday tool and as said earlier it has become uncool since parents and grandparents started using it (my friggin' dad, computer illiterate until 5 years ago is on FB and posting away like there's no tomorrow) but social media has stayed cool and will stay that.
FB is still too big to fail.. they are adapting to changing times but the glory days are over and the shiny paintjob has become a bit dull. People will still use it because they've invested much of their time in it and have connections there but it may be the 2nd, 3rd or even lower ranked sites people check when they go online.