This is probably related to the fact that this site usually offers value as in information being usable for practical IT related issues, which is encouraged by

This is probably related to the fact that this site usually offers value as in information being usable for practical IT related issues, which is encouraged by e.g. spam, ads or pointless clickbait articles being downvoted. Facebook is basically the opposite of that. So you might guess that in the context of Hackernews a site like Facebook could be the anti-thesis or enemy. Take it with a a grain of humour.