All you need is an imagination. Paid only-fans links for example would be a huge winner when targeted at virgin men in their 20s with no real life friends (AKA Reddit userbase). These subtle advertisements are already happening all over Reddit through user posts, it's just Reddit stupidly does not charge for it.

I used to have a similar impression. Gradually my view has changed. I now see it as one of the best sources of information on the internet on specific topics.

The question for an advertising business is how much do the people interested in those topics spend, and how hard are they to reach elsewhere.