Most of the websites, even messenger block such links and content as your messages are scanned in order to find some forbidden things. So I think it's only possible on related forums or in encrypted messenger that have no access to your data.
Ello.co does allow adult-oriented content - there you can find anything between artistic nudes and full-on PIV action.
The lack of social media for porn means that it's one of the few big markets where forums (XenForo, MyBB, phpBB etc.) are still hot.
Oh, and there's FetLife - it's a social network specifically built for those into BDSM, fetishism, and kink. It describes itself as "Like Facebook, but run by kinksters".
Since Tumblr Started banning them (when Verizon bought them) there is still: Twitter.com newTumbl.com Sexxx.com Pillowfort.io dreamwidth.org joinmastodon.org FetLife.com Reddit.com Sharesome.com Pixiv.com VK MeWe humblr.com kinky.business glitch.social motherless.com
why would social networking sites that can successfully promote anything they want, why would they allow that crap?
Top social sites don't allow porn. Because that's a very small niche and porn content are not acceptable for everybody.