Weird facebook/social issue

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Cliff Notes: I'm bored and I'm curious about a perplexing facebook blocking

This is probably a retarded post, so only read this if you're really bored, but...

I went to trivia at this local pizza place on Wed night with a bunch of friends. Of the 7 or so of us, there's one girl there who I faintly recognize but I don't know who she is. I find out that she goes to my school and we talk, but only here and there. Nothing out of the ordinary. Later a bunch of us go to another bar and play some pool, drink, etc. After that 3 of us end up at one guy's apt -- he's one of my better friends here so I was actually there mainly to drink with him and shoot the [censored]. Neither one of us was making a move on this girl so it wasn't like there was anything going on, we were just hanging out and playing drunken Jeopardy (ie, watch TV and call it out AFAP). I talked to her a little more while we were there, but it wasn't like we were hitting it off as far as I could tell. It was just because there were less of us there. So I bounce around 4 AM and walk back to my apt while she sleeps on a couch at that apartment (just so it's clear, there's a 0% chance there was anything going on with them, she was just a little too drunk to drive).

Now, tonight, I see that a random person added me on facebook. I check out the profile and recognize her as my roommate's ex (but they're still friendly), so I hit add, etc. I notice that our one mutual friend (my roommate) has a new profile pic so I click on his link to leave him some dumb wall post about it. Then I see the latest wall post on his facebook is from the girl from the night before. But... I can't click on her name and her picture is a question mark. I'm like, wtf? Out of curiousity, I try to search her and nothing shows up. Another buddy of mine is down her staying with us this week, so I ask him to search her -- I didn't think it was possible to make yourself un-addable to the entire community. She comes right up.

As far as I can tell, she blocked me. I have no clue why. Is this a technical bug of some kind in facebook? I find that hard to believe since it's a very well-programmed site. But I also can't believe that a girl who I don't even know and only hung out with for a few hours would block me. Not only that, she was telling me to go to another party that happened tonight (I didn't go). I wasn't getting bad vibes from her at all.

Really confused. I don't really need a play here because I don't care that much, but I'm really really curious wtf is going on here. Anyone have any clue?