I've had a lot of "weird feelings" about the visitors on HN over the years. Enough time elapses and you realize most of it is in your head and typically leads to a dark forest of subsequent thoughts.

This attempt to bind HN participation to employer is confusing to me. Is this not a (mostly) anonymous board? I usually have zero idea who anyone is working for at any given moment unless explicitly mentioned in whatever comment.

A more likely middle ground conclusion is that you are partially correct - perhaps most FB employees who post on HN just don't disclose who their employer is at the same frequency as others, so the aggregate perception is that fewer FB employees frequent this board.