Let me start off by stating that fraternities are not very common. At all. I think there might be ten at most (boys, girls or co-ed). If there aren't and I am way off, it just tells you how people around here don't really know or care about them. There are no "houses" for frats or sororities, so they are more like clubs than an actual fraternity/sorority.
There are many clubs, but many people do not care about them. I was "in" a club that is like an honors society, but we didn't do anything at meetings and frankly, I found it quite pointless. The only point of having the club was to have a club, and that was it. Plus, if you live on the Water Tower campus, it sucks to have to go to the other campus for nightly meetings, especially since most of them are on the Lake Shore campus.
Only in freshmen dorms do students leave their doors open, and only in Mertz is it really predominant. Regis, Simpson, and other freshmen dorms do not seem to have that unspoken open door policy, since people keep to themselves there. In the upperclass dorms, the isolation is predominant as well. In Baumhart, it is like a hotel, so no one leaves their doors open to the hallway, especially since every door on every floor either faces another person's room door or the length of the hallway or whatever, so it'd be like someone is watching you whenever your door is open. As for the other dorms, the people keep their doors closed and locked for the most part, since building communities on floors is more of a freshman thing.
Basketball games I would say are the only obvious athletic event we have here, and not many people go to them. Rarely will you hear of someone saying "let's go to the game tonight." A lot of people don't even know when our basketball team plays. There is little school spirit, regarding rooting for a team/mascot.
As a girl, I can tell you the dating pool is limited. If the guy is not gay, then he already has a girlfriend (either at Loyola or another school) and if he is still single, then it probably means something's wrong with him.
I met my closest friends simply from housing (my neighbors frosh year), or I already knew them from high school. Since I am in the city near several other schools, I often visit my friends who go to neighboring universities like Colombia or Northwestern.
If I were awake at 2am on a Tuesday, I would be wondering why I was up so late studying. I am not a drinker, so I don't go out to bars or anything. I guess any other person would say that they were up because they were at the bars, but I would be the one either up because I was studying, or because I was hooked on a DVD set of a TV series or because I was up chatting with roommates.
People party on the weekends, and you will see people in their "going out" clothes. There are a few bars nearby that surprisingly let in underage students pretty easily. You wouldn't expect that in Chicago, but it is very common for students to go to PCo's (The pumping company) and Hamilton's. Thursday is unofficially Thirsty Thursday, so it isn't uncommon for students to head out to party on Thursday nights.
Last weekend I went with some friends to Oven Grinder's (a pizza place by the location of the St. Valentine's Day Massacre), then rented a movie to watch with a friend in my dorm room. Saturday I did laundry and sunday I wrote a paper. It was a lame weekend, but for me (again, b/c I don't drink), it was pretty entertaining. Also, it is winter, so I don't really want to go out anywhere because it's too cold. Had it been springtime, I'd probably have been out walking Michigan Avenue or hanging out at a park playing wiffleball.
As for something to do on a Saturday that doesn't involve drinking, I am the one to ask since this is my forte. I would either have a party at my dorm room (like a costume murder mystery party) or see a movie, go to a restaurant, etc. There really isn't much to do that doesn't cost a lot of money and doesn't require a fake ID (like clubs).
Off campus, I hang out in the city, like shopping on Michigan avenue, or go to the park and take goofy pictures with friends. If it is nicer weather (above 65 degrees) I'll walk around and just hang. There are a lot of touristy places people are obsessed with seeing if they go to school in chicago, but the less-obvious places are fun too. This includes the Belmont shopping area (off the Red Line stop on the El train), or even taking the El to a random destination and walking around to see some funky shops. There is a coffee cafe in this park near the downtown campus called Whispers Cafe that is nice to hang out at when it is nicer outside.