I don't buy that is a monopoly. The networking effect alone is not enough to call it a monopoly, and I think the natural end state is social networks with interoperable APIs. It is just a natural opportunity cost with a high valuation since it requires a core peer group to follow, but nothing prevents you from building a competitor. You would have a slow adoption rate, and would need to compete on features to attract the nerd crowd, which would be followed by their friends, and in a delay, the peers and family of them, but I think the market movements in social networks are more comparable to real social and cultural shifting rather the instantaneous results entrepreneurs expect.

I don't think in a decade the closed social networks will last, for the interopt reason. The value proposition of a common platform of profiles is too strong long run not to take over.