I think there's a big difference between making the site more active and bringing back old users. Chances are that many old users that left left for a reason. If it's an external reason such as work, studies, new hobbies or whatever there ain't much we can do. If there's an internal issue we could try to fix it. But more importantly I think we need to get new members. If you can come up with any ideas for that I'd be interested.