I would say that reading a person's entries is what gets them on my mental network. Sustained commenting from them to me might do the same, but I don't recall that happening very much, and a single random comment is unlikely to have that effect unless the person is popping up a lot in other people's journals as well.

The problem with this, of course, is that I have to remind myself that these people, about whom I may know quite a bit because I've been absorbing what they post, have no fucking clue who I am or why they should care. It's hard to remember that when you feel like you know them.