I think grandparent's point was that the first cause you mention - reducing FB's power over users - does nothing for the users if this power instead goes to other 3rd parties, and perhaps even grows.
GP points out that in a distributed social network, 3rd parties can still mine your data, you still have trouble permanently erasing information, and in fact these problems grow instead of shrink. From a user's POV, what matters is the total amount of 3rd parties over them and the leverage they have against these 3rd parties as a whole. GP's point is that shrinking FB's power by going to a distributed social network might actually increase the total power 3rd parties have over users.
GP points out that in a distributed social network, 3rd parties can still mine your data, you still have trouble permanently erasing information, and in fact these problems grow instead of shrink. From a user's POV, what matters is the total amount of 3rd parties over them and the leverage they have against these 3rd parties as a whole. GP's point is that shrinking FB's power by going to a distributed social network might actually increase the total power 3rd parties have over users.