It seems like there is a lot of focus in the meta about recruiting new people, but it doesn't seem like there is a whole lot of returning participation. Many of the questions and answers are either written by one of a few frequent users, or by new people.
This SE could probably reach critical mass just by trying to get people who have already discovered it to return. I'm a good example of that. I had used this SE to ask 1 or 2 questions, but never really touched it. It was frustrating having very little rep because I couldn't comment or anything. Then I got my StackOverflow association bonus and could actually do stuff here, so I came back.
I don't know if there are stats on that, but it seems like it would be worth the effort to recruit people who have already been here to come back.