I lost 30 pounds in college drinking (heavily) 3-4 nights a week. I would never drink "casually". It is crazy, but I love to drink. Now I only drink heavily maybe 1 night a week, but I drink casually from time to time during the week. I am 22 now and happy just hanging out with my boyfriend, I was 19 then (don't call the police!!!)
I lost the weight by doing Tae Bo on days that I was going to drink. Tae Bo burns as many as 800 calories in an hour. 800 calories is about 8 light beers, and since I had to walk to all my classes on campus, I was active anyways. Also I was lucky I never got hang overs (I do now!), but when I feel like crap from being hung over, I force myself to exercise because it forces me to drink water instead of sit around and eat all day, which is always great!
But my advice is simply weighing your desires... you can be skinny and drunk at night, as long as you are pretty strict during the day.