I don't know, Reddit has a lot of artrosurf. You can buy up votes and down votes very easily and you regularly see suspicious votes on some content.
Regardless, voting is just one check on bad behavior. The other one is moderation, and at least on the subreddits I visit, moderators are strict on financial self promotion, but it can still vary.
Even on Hacker News comments there's people talking about "hey, I'm working on a startup." and someone asks "what startup?", and they reply "here it is." with a link. I've rarely seen those exchanges get downvoted.
Oh, it's very, very obvious, and Actual Human commenters all ask "what is this doing here?" And that style of message board spamming is decades old, so it's about as obvious to me as a flashing gif banner ad.
> The other one is moderation
True, once the mods wake up, the posts are always removed. But arguably the damage was already done, and like I said, if this many obvious manipulated posts slip through the cracks, we can make some statistical assumptions about the presence of more evolved predators.
Anyone that starts pointing this out gets shadowbanned or downvoted by the herd.
Which content did you have in mind?
Also some early vote manipulation on the new submission can have a lot of impact using very few accounts.