I think Tumblr did a good job with this when they created the "Blaze" thing. For like $15, you can "blaze" a post, which will put that post in an ad slot for like 100 people, and you can pay more to show it to more people. This basically allows Tumblr to run with reduced dependency on actual ads while allowing people to force whatever garbage post they want into a random selection of users' feeds. The real kicker, though, is that you get no say in who your post is shown to - no demographics and Tumblr has no algorithm, so it's entirely random - and you can blaze other people's posts. It's like a chaotic evil version of c/CuratedTumblr or c/MicroblogMemes and very much in the spirit of Tumblr's userbase.