Where are you seeing this happen? I can only think of one social media website that doesn't easily facilitate anonymous commenting: Facebook. Believe it or not, they sometimes flag new accounts and make you submit photo verification (happened to me).

Twitter as well. Try to make a new account in an incognito window without providing your phone number and you'll either be stopped from completing initially or find the account blocked before you've interacted in any way.

And if we're talking about real anonymity, lots of the internet is blocked off if you use TOR, whether by the site itself or some cloudflare or whatever CDN or hosting service.