I think the best party time at SIUC is unofficial Halloween. I think that because it is usually still warm out, everyone is out and it is a very fun time. The bars always have specials and competitions going on. The whole atmosphere of the strip lights up and everyone has a great time.
The most popular student groups on campus are probably the various fraternities and sororities, although I will say that Greek Life is less popular in Carbondale than at other schools. At many Universities you might find that frat parties are the most popular spots to be, and depending on who you are and who your friends are this might apply to you at SIU, but speaking generally the local bars are much more popular, as are non-student organization affiliated house parties. The best ways that I've found to meet people at SIU (something that I also think applies to any college) is to simply join a club or student activity group. Joining a club is a great way to meet people with similar interests as your own. Also, being at college is an amazing time to expand your horizons and try new things, so I would even recommend joining a club related to something you've never even done before, just to see how you like it. Just to list a few, there are many sport clubs, (i.e. Rugby, Baseball, Fencing, Skydiving, Ultimate Frisbee), and there are major specific clubs like Sigma Tau Delta, which is an honor society for English majors. There are other major specific honor societies that I would seriously recommend any aspiring scholar to join, because not only does it provide you an environment to expand your mind and meet other intelligent people with similar interests as yourself, but it also looks great on a resume, which is something you should always be working to improve. Also, becoming an active member in whatever student group you join is important as well, because it will show future employers that you take initiative and can handle responsibility. As far as what you can expect dating wise, I'll simply say that you can find people you're compatible with and enjoy just about anywhere, and SIU is no different. That being said, what seems to be the case at SIU, is that a lot of freshman continue dating their high school boyfriend or girlfriend well into and many times throughout the entirety of their freshman year, and sometimes further. This is not unique to Carbondale. I know many people who have very serious relationships in college, at SIU and elsewhere, and I know people who aren't exactly limiting themselves to a single relationship. Carbondale is the kind of town where you can pretty much find whatever you're looking for, you just might need to look particularly hard depending on what it is. Not every guy and girl is looking to be tied down, and some people are only looking to be tied down. What I will say is that college is a time when you can and in most cases should try everything you want to try and meet as many people as you can, because you'll never be in a place again where 15,000 people your own age are all living together in the same town and all looking for something to do on the weekend.