Besides the obvious remarks about people not caring a lot about their privacy and not wanting to pay a cent for a social network the biggest problem is a coordi

Besides the obvious remarks about people not caring a lot about their privacy and not wanting to pay a cent for a social network the biggest problem is a coordination problem: Facebook satisfied a nearly universal desire for online social interactions and as a consequence gradually all your friends joined the network. That was something new and exciting. Replicating this huge social pull effect once more is incredibly difficult - you'd have to create an entirely new kind of social network.