I could go into a bit more philosophical angle with reterritorialization done by capital, but I think it's much simpler to consider the following: between politics (where once any division in population is found, it becomes valuable instantly), classical business marketing (where once anyone makes purchasing decision informed by something, it becomes valuable), capital markets (where once anything can be used to predict anything about any company or asset class, it becomes valuable) and more personal scams (where once you figure out someone's niche interest, it becomes valuable) there just isn't anything left. Go ahead, try to find something.
Reddit is being constantly targeted. I still use it, because what else?, but if the method isn't obvious, here's what you do: you are hired by/own small company making niche potato chips dip sold via Amazon. You go to google keywords and check 'potato chips dip', you google all you can find there and some of your ideas, you write down all top10 results and check every now and then (well, your SEO monitoring app does it for you). Whatever you find that allows for user input, you generate that input - accounts are cheap - and maybe do some external SEO (thus beating 99,9% of social media results online).
That's it, it's easy. What Reddit (and any mildly aware SM company) does, is they try to offer marketers access to audience for a price that's lower than cost of what I just described. There will be edge cases, especially on international markets where value of time for various business owners differs vastly, which will lead to sites slowly getting more clogged up with ads, but that's the general gist. If you can imagine using similar method to get in front of your eyes when you're looking for something, then it would just be quite weird if nobody ever did it.