I don't do much of it. Possibly, because I really have to know a person or at least see her or him live to post an adequate comment, which will convey an adequate reaction.
A little advice to fellow bloggers: never post a comment like "I don't want to know about this" to somebody you do not really know in real life. Nobody forces you to read blogs or comment on them. And for some people its really hard and scary to write anything personal. The comment like that will really hurt.