Other comments already pointed out the use of "nofollow". But it is also worth a mention that old, well established sites of high rank link to Hacker News. Most

Other comments already pointed out the use of "nofollow". But it is also worth a mention that old, well established sites of high rank link to Hacker News. Most link farms do not have incoming links from highly ranked sites, so the existence of such links is another clue that Google uses to distinguish a discussion forum like Hacker News from a link farm.