They just say what people have said offline in the past.
No different, really, except that everyone has the ability to hear what’s said, as well as make a counter-argument.
The case that comes to mind is a discussion I participated in in which a young photographer was called a hoax by a blogger and I called them out on it:
http://www.beikey.net/mrs-deane/?p=218
Bloggers who make too many baseless accusations are eventually regarded as idiots.
It’s better this way, really.