Unlike chronological internet forums where all opinions must be heard, it's very difficult to successfully astroturf an internet forum that self-polices with upvotes and downvotes.

EDIT: In the case where someone says something against common perception, downvotes in particular deemphasize it (turning the comment gray on HN, collapsing it on Reddit) as this comment unintentionally demonstrates, thus why astroturfing in such forums successfully can be hard to do nowadays. The real astroturfers do Facebook/Twitter instead where content can't be downvoted.