If a tweet gets shared enough, its author gets approached by brands who want to pay them to add ads below the tweet. The same probably happens on Reddit, except it's even easier, because anyone can write a review, since the author of a popular post doesn't have a visibility advantage when posting a reply to it.
Just find some random users, and offer to pay them for a good recommendation on some "what is the best product to do X?" post.
When the results degrade, I'll stop using this approach. This feels like an "argument from cynicism" given that there's a widespread feel this gives better results (and intuitively, this feels like microbartering and more as likely to get a reddit post about the phenomenon as acceptance, no not everyone will accept random $100 solicitations, and they're not influential enough for more)
Just find some random users, and offer to pay them for a good recommendation on some "what is the best product to do X?" post.