A helluva lot of people joined these networks and got captured by the network effect because we were told or believed that they were open sites, just like the rest of the net. If content (or types of content) came along that folks didn't want? Nerds like us would make and give away (or sell) tools for getting rid of that garbage.
In a big way, what we're going through right now is fraud, plain and simple. It may not meet the legal definition, but to grow and scale using one model and then switch to another once you have a monopoly is a form of bait and switch. There's no way in hell I would have joined either site, and I would have vociferously told my friends and family not to join either, if they had been clear up front that this was what they were planning to do.
NOTE: I am not making a legal claim. I mean fraud in the moral sense. I understand with a zillion corporate lawyers that everything these guys do is wonderfully legal, no matter what the long-term social effects end up being. I am speaking in the vernacular, not legal sense. We were tricked.