An alternative would be to charge users for entry to the site. If the charge were set low enough it might work. But I don't know how many people consider social media sites to be enough of an enhancement to their lives to be worth paying for. It's hard enough to get people to pay for music, art, movies, books, etc. Also by charging a fee you'd be skewing the demographic towards wealthier people, which would suck.
I think it is time for a pay-social media site. There was one on Kickstarter that I almost funded a couple weeks ago. Ah! I found it. Here it is: OpenBook I'd also like to see a pay model that incentivized participation over browsing. A "like/heart" would cost you a single Dingleberry, and and downvote would cost you 10, but a 1,000-word article with original images would award you 10,000 Dingleberries. Or something like that. Make the base price like $10/month, and you weed out the majority of trolls. Then you incentivize participation, provide up/down rankings, good content moderation tools (flag/edit/delete comments on your content), and an anti-griefing system (like Hubski) that neuters the assholes, and that would be WORTH $10/mo. Could work... for a while... until r/The_Donald found it.