I can't see either vanishing.
People love FB. If it goes away, then all that means is G+ stole the market and that will only happen if Google overtakes MS as the default operating system worldwide.
MS is still extremely dominant in every market. Only way its going dodo is again.. Google and its ChromeOS or similar.
However, MS is smart enough to adapt to the cloud technology / base that is central to Google's platform. So in the end, they may lose marketshare and never really gain a big foothold in mobile space, but they won't vanish.