I think its rather early days to reach this conclusion. A social network is just a network of people. In the real world small networks of like minded people have often had a disproportionately large amount of utility. The one thing for large social networks right now is that they provide a global namespace of people which helps discoverability. But perhaps in the coming years we shall see social networks emerge for vertical talents where likeminded people can work productively (Mendeley etc come to mind). You could arguably make the point that within each of these verticals a single player will dominate.