I think companies should only be able to target by site. For instance if you sell tires, you should buy advert on a car forum.
All other forms of targeting is a manipulation and should be illegal along with any form of tracking.
If you want to increase awareness of your innovation, you might want to target exactly those people who aren't car hobbyists and don't visit car forums. Same if you are selling a product specifically designed to be easy to use for non-experts.
Ads on Google would only be about web searches generally? Like crawlers, and database performance?
Instagram should only show ads for photo-taking equipment?
Fostering a community is hard work... kicking out spammers, fraudsters and peddlers of illegal material (cp and DMCA), dealing with trolls and shills, directing newbies to useful resources for example. Without moderation (or too lax moderation criteria) your forum will end up overrun with legally questionable material (4chan, 8chan, 8kun), alt-right conspiracy theories (Parler, gab.ai), porn (r/worldpolitics) or questionable "advice" (r/legaladvice).
The problem is that moderating content is rarely positively rewarded - HN with the constant praise of u/dang for their moderation work is the example proving the rule. You're too lax? Users complain that the community is "devolving" from its purpose. You're too strict? People will raise off-site shitstorms or run organized brigading attacks.
This is why group admins deserve to be compensated for their efforts (and in the case of FB mods, way more compensation given the constant onslaught of cp, gore and other extremely vile stuff).
Also it incentivises moderating groups rather than just letting them be and doing the bare minimum.
I still don't like the idea but entirely for the reason that it adds financial incentives to demand moderators force groups to be advertiser friendly in more ways than just removing toxic content. If advertisers don't like something it will simply be neglected in comparison to the more advertising relevant ideas.