Ten to fifty years is a long time to project into the future .... I think there will not be so much a replacement social media web site to Facebook as 1) fragmentation of the social media space. 2) the waning of social media as the predominant focus of personal computing. I would go out on limb and say that software agents will be the next big thing. That is applications that know a lot about you and do things on your behalf without your direct intervention. Less interaction, not more. I think the proliferation of web APIs and big data, make software agents more viable.