Seems like it could be fairly simple to define whether a UGC site can be considered "not social media".
* 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.
I offer a refinement on the 'targeted ads' aspect.
No USER targeted ads. Ads based on page content should be allowed. I say this as someone fully against ads as a concept entirely. They are not helpful nor a benefit for consumers. Consumer awareness and product fit for purpose tests should be funded and expressed in other ways. Maybe a competition (think 'buy ins' like poker tournaments) as an idea / one example.
Maybe page content and location, or at least location in a broad sense (down to a city or state level) to allow for small niche businesses to buy relevant advertising. I.e. The local model train shop buying ad space for users of a model train forum that are conceivably close enough for them to actually go there
Location data seems like a better fit for maps, which would tie in to searches for X, and maybe include some sort of coupon that when used also gave a small kickback to the route which the customer got it via.
At this point we're no longer talking about social media we're talking about personalisation. None of what you said touched on either the social aspect or whether something is or is not a medium for such activity.
Which is fine, but probably something to keep in mind when debating definitions.
Most of what you mentioned is more about privacy. Which could indeed do with some additional protections, though attempts at an age restriction tend to run into the paradox that you need to know someone's age to enact the restriction, which means you need their personal info, which is what we're trying to avoid.
I'm not sure preventing targeted ads or recommendations it is enough though. Part of the problem with social media is that they accelerate all types of social interaction, which may be harmful in and of itself (you don't need recommendation algorithms or targeted ads for something to go viral after all, viral memes predate both of those).
* 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.