At my college, the student body was about half commuters and half dorm residents. Those of us who lived on campus didn't really know most of the commuters, as they would leave campus as soon as their last class ended. A handful did hang out on campus, though, and the resident students came to know them very well.
Those commuters were also in student organizations, went to intercollegiate sporting events, chatted in the student union (chatted meaning in person, no internet back then), and so forth.
Another way to socialize in college is through studying together. Ask some classmates if they want to go somewhere for a study group. Go in on a pizza, and find a public area or an empty classroom to study and socialize at the same time.