I don't know about Reddit but we see comparatively little on HN that appears suspicious once we look at the data (e.g. which users have voted on a post). Anyone who suspects astroturfing on HN is welcome to email us so we can look into the matter—which we always do—but not to accuse other users directly, because both the odds and the cost of an unfair accusation are much higher than people realize when they do that.

I listened to that talk you linked to. At the end the speaker mentions four "hallmarks of astroturfing": (1) use of inflammatory language; (2) use of charged language to "debunk myths"; (3) attacking an issue by controversializing the people around it rather than addressing the facts; (4) reserving all public skepticism for those exposing wrongdoing rather than wrongdoers. It seems to me HN is in pretty good shape here: the first three violate the site guidelines, and the fourth seems rather rare and is not received well by the community.

Maybe there are astroturfers getting away with it on HN. If they exist, though, they're being clever about it, so we'd be interested in anything the community can figure out. Just please don't accuse each other directly without real evidence.