not OP, but I would think that a core aspect of it is having a social graph, i.e. being able to follow specific people, and having other people follow you. Thi

not OP, but I would think that a core aspect of it is having a social graph, i.e. being able to follow specific people, and having other people follow you. This is true for twitter/facebook/friendster, but not for hn/reddit/slashdot. Some of the negative effects of online communities are shared by both kind of things, but not all.