Facebook has sucked for a LONG time now and even though it's losing people, it's still huge. It'll be around but continue to shed customers to other social media websites until the big new thing comes (ie. Facebook c. Late 2008-early 2009, when it effectively replaced MySpace).
I'd say Instagram.
Youtube can be replaced by music streaming services that starts to offer videos
Facebook is already fading, I guess
Twitter can be incorporated into another site
youtube. I can actually see facebook being replaced by something else, its popularity has plummeted with younger people and there's the whole gathering (and selling) personal info thing
