I think I am pretty fortunate, I have never fallen foul of mental health issues (at least I don't think so!).
I do have a somewhat addictive personality, and came very close to developing a dangerous cocaine habit in my 20s. It just happened that I was around and the right age at the birth of acid house and the rave scene, and I had a lot of fun at various illegal parties and with the drugs that came with it. I am not saying this to glamourise either it or me - hell, drug use is pretty seedy in general - it is just part of my past. Anyhow, a pitying look from a good friend that I was not meant to see but did gave me the wake-up call that I was already sliding on a very slippery slope, and I managed to step away from something that could have been catastrophic.
I have had my moments when gaming - Dark Age of Camelot and WoW properly hooked me in back in the day. Generally though, gaming is my downtime where I can switch off my brain and not let the stress of work persist once I am home.
I think that the advent of the internet will change social interaction more yet than it has already. As others have pointed out, spending time getting hammered in way overpriced pubs and clubs is a waste of time, money and brain cells. While I do like to get out for a beer and a meal from time to time, my preference would always be a good single player or co-op game with friends.