The easiest way to make a social circle, or get into one is to join a sports team around campus. You can do one of two things; if you don't have a team, either make one on your own ( this'll prove you got balls to initiate), or join a team that needs a ringer in whatever sport. If you are any good, teams will be in line trying to recruit you
One you prove you can carry your weight, have some fun, suddenly you'll start going out to celebrate victories, and meeting new people. In two years, I joined two different teams and made good friends with very established social circles that I otherwise would not have.
Other things I did was get jobs at the recreation dept. I reffed on some nights, which not only broke the tedium of my work, but it opened up a new group of people I otherwise would not have met in my major. When you are in school as long as I was , social circles come and go. You have to learn how to become a focal point for a group of people for a common interest, and naturally people will percolate into your life.