my bet was predicated on (1) people realizing the insidiousness of facebook, (2) the cool kids moving on to other platforms, and (3) mobile communications being omnipresent and eclipsing facebook (solid but not earth-shatteringly original reasons).
i'd attribute the demises of friendster and myspace primarily to #2 (with underlying product and marketing issues exacerbating it). but more than that was required to topple the massive network effects moat that facebook built. notice that it wasn't another social network (e.g., snapchat) that will have overtaken it but rather a confluence of other issues striking in concert (some self-inflicted).