I know there are a few gagers who have left for personal reasons but do plan to return, such as @MissNowhere who I think is taking time off to concentrate on herself and her studies. I think others have just grown tired of frequenting the site and don't feel like they fit anymore.
Some of them have while so many of them left because the website went downhill last semester. Recently it has started getting better again. However you still can't get any answers to good/important questions still.
Erm... A lot of serious questions go unanswered while other questions that are pointless, or based around sexual themes are quick hot topics. I can see the annoyance in this and even thought of leaving myself because of it. Not to mention, the rude opinions some people post.