Yeah, I am also astonished at the nasty comments on FB, I think a lot of them are from young, immature people who sadly don't realise their comments are up there for all to see forever! I either ignore them or post a positive comment in reply, some comments are so nasty they are just not worth even thinking about, but it makes me wonder how many insecure, immature people are out there in FB land, it's a worry.
It is so tempting to post a comment pointing out that their nasty comments will be online forever and that prospective employers may see them, but I suspect they wouldn't even care ... And I suspect Karma will come back and bite them on the bum one day!!!