In general, MD/PhD socials are your chance to get a feel for the program and its current students. It is also the program's chance to impress you with its generous bar tab allowance. Generally, you will talk about your research interests for part of the night, and conversations will drift from there. When I was interviewing, I went to socials where the conversation covered everything from cheap European travel to the TV revival of American Gladiators. It was wide open and faculty members generally were not present unless stated beforehand (dinner at the dean's house, etc).
So, when it comes to the socials, just relax and be normal. This sounds easy enough but I have attended socials where other applicants chose to talk about their MCAT scores or whine about their lone B+ from undergrad. Do not be that guy/girl. Everyone will hate you and your fellow interviewees will be tempted to tell stories about their trips to Tijuana just to shut you up.