Agreed, but note that for a lot of people on Facebook, their primary audience are friends, family & acquaintances; people who will mostly tolerate your (contrarian) views, whether you display tact or otherwise. People are the center of conversations on FB.

This contrasts with a place like HN which has no concept of friends and family -- the center of everything here is the comment box. That's a great trade-off as it allows us to focus attention on debating contrarian ideas, and less on the one who is airing it.