My girlfriend's roomate says she has a crush on me

I was at my girlfriend's apartment a week or so ago, when her roommate told me she had a crush on me.
I didn't tell my girlfriend about this, which makes me guilty because I think I like the idea of someone finding me attractive. Should I tell her this?
You should, if only to avoid her finding out about it from her roommate at some point. If that happens, she will wonder why you kept it from her and you can simply avoid a potentially awkward situation by talking about it now.

My friends got into my Facebook account when I left my phone unlocked and sent some random girl messages about how much I care about her. She responded and I was surprised when she said she liked me too. We were friends online, but I had never looked at her that way before. We went out on a date and got along and she thinks we should consider getting into a relationship. I don't want to tell her that it was my friends who sent her those initial messages though, because she will think of me as a liar. At the same time, if one of my friends eventually tells her at some point, I don't know how she will react. I'm scared to just be honest with her. What should I do?
I think you're overreacting a bit. If you tell her it was a joke, she might just accept it and move on. The important thing here is that you are both in a relationship that appears to be working, and the foundation is set by how honest you are about things. Tell her what happened, explain why it's awkward for you, and you can both put this behind you and concentrate on more important things. To obsess over it and wait to tell her about it a year later only makes things worse. I should also point out that it doesn't seem like such a big deal at all.