Why did her friend add me on facebook?

[WhoDat!]

Granted this girl knows the rest of my roommates in my apartment.... I was walking back from a different apartment building on my campus last weekend when I ran into this girl. I was a little drunk when she got on the elevator with me. We were the only two on there. She looks at me and asks me if I'm this one kid's friend. I said yes and we continued talking as we walked back to our apartment buildings. Then we separated. I added her on facebook shortly after I got to my room... then the next day her friend/roommate added me on facebook.

I'm a junior and she's a sophomore...