In any case, I was (while avoiding NaNoWriMo and homework) flipping through my friends list and Google feed-reader, and thinking about why and when I comment on people's posts.
There are a few people (2, I think. Maybe 3) to whom I almost always respond, regardless what sort of post they make. And these people are ones I would consider actual friends, or in the process of becoming so. We have similar views on things, like to talk about the same stuff, and I know they won't be offended if my comment is a bit weird or incoherent.
Then there are a whole bunch of people to whom I sometimes respond, depending on the sort of post, the mood I'm in, and various other factors. Some of these simply write stories I like, some are people I'm interested in getting to know better. (Some are simply highly entertaining in their freak-outs over real life -- and I'm not saying anything more on the matter.)
Everyone else falls into the "I'm not saying anything unless they write a brilliant story (particularly if no one else comments on it) and perhaps not even then" category. For which I sometimes feel guilty -- there are several people who write very, very well and are fairly prolific, and yet have received (perhaps) one comment from me. I'll say to myself "You know, I should really let the author know how much I enjoyed that", and then I just -- don't. And I'm not sure why. Habit, maybe.
How about you all? Why/when do you comment on stuff? If you don't feel comfortable admitting stuff in public, go ahead and comment anonymously -- or don't. After all, that's a kind of answer in itself.