* No recommended content. Chronological feed with basic search functionality is okay. Auto-generated "popular posts" section is social media.
* No targeted ads. Static banners which everybody sees are okay. Decision-making about which ads to show based on user data is social media.
I think those two simple rules would keep a "not social media" company's incentives aligned against invective and towards relevant discussion, even if they were in the business of facilitating discussion amongst like-minded people. And it's not like social media would be banned: just more tightly regulated.