If you think GPT-3 is scary imagine the kinds of shit governments and corporations have hiding in private for their own use.
"Do bots exist on social media" is an answered question. Yes.
On Reddit at least most of its "front page" is manually curated garbage to create specific perceptions in peoples brains. Shit like divisive politics, anger, hatred, smugness, etc.; Its a disgusting website.
Twitter Inc. of course manually controls its "trending" bar thing, so wrongthink topics never reach any significant levels of visibility. If you say something bad Twitter locks your account and forces you to delete your own tweet, a truly humiliating act that exists solely to condition you in their favor.
4chan is a gateway to more imageboards and will forever be the most popular English imageboard. A lot of different groups/governments/people/etc. want to control your thoughts, especially on more open, free-speech oriented sites like 4chan. If bots are used to shit it up with extremely low quality, single word posts, then it pushes away deep conversation and innovation.