> I see how manipulating opinion on social networks can yield a gain. But how would entity X profit from manipulating opinion on a small community of tech enthusiasts?

Having your posts yelled at and downvoted can reduce your willingness to participate in discussion or debate. Especially when there's a stupid "karma" score that subconsciously gamifies our participation here.

I'd be willing to bet that having one's opinions silenced online trains one to stop talking about the same topics offline as well. Like a sort of conditioning.

And there are lots of powerful parties that want to silence certain opinions.

Why on HN? Because of the abundance of smart, wealthy, influential, and well-connected people here. Change these people, and your social engineering ripples throughout society.