Run the service as if it was bound by the First Amendment (i.e. only illegal content should be removed, basically how every social network ran prior to a few years ago.)
No large social network ever ran that way and I'm skeptical that any social network could be run that way and make a profit. Here are some of the things that are not actually illegal to publish under US law:
- Most forms of doxxing
- Adult nudity and pornography
- Most gore images and videos (industrial accidents, car crashes, corpses from war zones (including children), animal abuse...)
- Praise and promotional material for pedophilia
- Praise and encouragement for anorexia and other eating disorders
- Promotional material and instructions for suicide
- Praise and promotional material for terrorist groups like ISIS, as long as it doesn't call for "imminent lawless action." Much historically banned-by-bigtech ISIS propaganda was not actually illegal, which is why government officials urged companies to do something about it instead of just dispatching law enforcement.
- Spam that isn't delivered via email
- Coordinated foreign propaganda, e.g. "50 Cent Party" commenters piling on to criticize anyone who criticizes mainland China's government
- Jailbait images (in the sense of the banned-in-2011 subreddit r/jailbait)
A lot of things that 95% of Americans never want to see online are actually legal, just so widely reviled and moderated against that you rarely encounter them accidentally.