I'm not sure any network can grow explosively without having the users submit their contacts from a previous network (email, facebook or phone, for example). And the "ethical" competitors don't want to do this, so they can't compete.
Right, but facebook isn't about great technology or user experience, it is about a common watering-hole. Claiming it is a "Facebook replacement" is premature until the adoption issue is solved.
The critical mass problem is the most difficult social problem to solve. Just have to make something people want to use and if they want to use it, they'll share it. That's the idea anyway.