Network effect, (I.E., all their friends/family are on surveillance social and so that's where they are), switching costs, (big social makes these really high, you can't move your network and you can't follow your network from small social and you can't migrate your content), and finally the UX tends to be easier for users on big social. All of this will outweigh privacy concerns as long as people can't quite wrap their heads around all the implications involved.