The word you are looking for is retention. You have to work on improving it. The basic idea is to get user's email address, twitter handle etc and remind him to come back to your site every now and then. You have to be careful with this though, it's quite easy to be labelled as a spammer if you don't do it right.
That would be fairly easy for us as we have "wantlists" and we do a good job (i think) of getting user information in the interest of establishing trust.
