> Social media doesn't have to be that expensive to run. Countless forums out there for decades.

Said forums existed because of volunteers paying in the form of time. Moderation is expensive, so are legal liabilities and associated cost that have only increased over the last decades - DMCA, anti-CSAM legislation, anti-terrorism legislation come to my mind primarily - and especially, there is a huge workload to deal with abusive behavior from unrelated third parties: skiddies, ddos extorters, dedicated hackers hired by "competition", spammers, you get the idea. Someone always pays the bill.

There is a reason so many forums and mailing lists collapsed once Reddit took off. It just isn't worth it any more.

It helps a lot for a community to have a specific focus.

For instance if it is photography technique or sports talk or Arduino programming almost all problematic content is "off-topic" and easy to delete without splitting hairs or offending libertarian sentiments.

Similarly "no explicit images" is an easy line to defend, but anything past that like "no CSAM" is excruciatingly difficult.

For a general purpose platform where people can post what they want, particularly if there is a libertarian ethos where people cry about "censorship", moderation is a bitch.

My personal pet peeve is that on any platform that has DMs I get a lot of messages, particularly when starting a new account, for things that are transparently scams and if I was starting one today my feeling of responsibility leads me to the conclusion that I would not support DMs.