That's very different though: porn and downloads you can get from any site. But for social media to work, by definition your peers need to be on the sites as we

That's very different though: porn and downloads you can get from any site. But for social media to work, by definition your peers need to be on the sites as well. If you apply that to porn, you end up with a handful of sites that have a very large userbase, and you can easily regulate those, because they're also very valuable. Now, if such a step would lead to large-scale adoption of decentralized social media, that might be another issue, but as adults wouldn't be affected, I doubt they'd invest too much time into it (+ many of the people who are capable of building it don't believe that social media is a good idea).