It's essentially the only way they can make money, unless someone in the future invents a creative new way to monetize social networks.

I don't like the way these sites try to own, brand and market various aspects of their users' identity in such a strong way, and make it hard to nigh impossible to archive or remove your data, but I understand why they do it. The only ideas I've had amount to "x with anonymous posting and a big red DELETE button" or "x, but charge for y and somehow convince people to pay or y even though y is free everywhere else."

I don't think I have a good solution but given the general growing unease and distaste people seem to have with social media, I know it's needed.