Personally, I don't see it happening the pull of a "free" service will always bring in people to use it and return, and the network effect is very strong, right now social media growth in the west has peaked out but its moving into other parts of the world who are subsidised by the west users who fall for ads and have money to spend

even if the profitable audience is in decline, many of these sites have build high margin businesses that are only really bloated by staff and their need to spend capital to avoid taxes, if a crunch comes in they can always kick out alot of the useless staff and find a new healthy medium