The percetnge of the population that even has a social life is small. Within the first couple weeks of school you will either know personally, or recognize the face of every social person at CMU, or at least definitely in your year. At the same time, when you find that group, you will definitely have fun. There's a ton of fun people, some on greek life some not, who love to party and go out to the bars just as much as any state schooler does. At the same time though, you won't find much going on during the week. Everyone here works their asses off Monday - Thursday, and then the social people party hard on the weekends.
I'm involved in greek life and it pretty much dominated my social life. I met my closest friends because we lived close to each other in our freshman year dorm, and we all ended up joining the same sorority. Carnival is the highlight of CMU, and basically serves as the homecoming for alumni. It's a long weekend full of debauchery. People aren't huge party animals, but you can have fun if you look for it. The administration has been clamping down on the fraternities, so a lot of underclassmen now have fake IDs and head to bars on the weekend, to the upperclassmen's dismay.
You work very hard, and if you're awake at 3am on a given evening, chances are you're working still. The payoff is rewarding, but do not come to CMU if you do not want to work. Athletics are not popular; they are getting better with each year, but it is not a school intent on school spirit. Greek life provides a welcome retreat for those that wish to have a social life, but is very easy to either be Greek on campus or not: no pressure with either and every Greek has more non-Greek friends than Greek ones.
Not everyone here is a nerd like most people think. What seperates CMU from other institutions is that the students know how to buckle down and study but they also go to parties and have fun. I once hated Pittsburgh, but it has grown on me. Everyone calls it a "fake city." I agree with that sometimes, but there is a lot to do. All the sporting events - Pirates, Steelers, Penguins - are easily accessible by bus. There are some great restaurants and shopping too. Definitely not just a college town.
Lots of leaders on campus, so the clubs are really active and do a lot of cool events. There's an Indian club that puts together amazing parties with great food and amazing dances every year. They also do this one event where people spray paint all over each other. The A Capella groups put on lots of concerts. The frats host campus events like a huge BBQ every year that is actually really good, etc. All sorts of fun events: hot dog eating contests, water slides, white-water rafting...
The Greek community is pretty small, less than 20%, but extremely visible on campus. Greek organizations are always holding events, social or philanthropic. Most of the parties during the weekend are at the fraternities. There aren't too many wild parties during the week because everyone's doing homework. There are great university sponsored events (movies, games, actvities) on Friday and Saturday night if partying isn't your thing.
The social scene is often mocked by CMU students, but it is what you make of it. If you want to party, you can find a place to party. If you just want to hang out with friends, there are movie theaters, sporting events (Pirates and Penguins games are pretty easy to get tickets to), bowling alleys, bars, some pretty darned good restaurants, et cet - all in Oakland, Squirrel Hill, and Shady Side.
people party like they're in high school. every party you go to someone will throw up and you'll see it happen. it might even get on you. If you're a guy and you don't know who's house it is or anyone in a fraternity, plan on getting kicked out, unless you're with cute girls. if someone doesn't know you at a party they will consider you an intruder-- plan on pushing and shoving.
ASA is a huge club on campus, so is Mayur SASA. Dorms vary by people, and there is a great amount of people that are nerdy CS majors. Freshman year-most boys stay in the frat quad. So, do the girls. Eventually, they will get ballsy enough to grab a fake and head down to PHI. Unfortunately, most bars card, so the chance of leaving the greek quad is iffy.
Athletics are a joke here but they're fun if you're on a team because they're low key. Fraternities are alright but commonly get the shaft by the administration when it comes to having fun. Sororities are in general over active ugly girls but there are a couple good ones.