Should I try and pursue this girl again?

Okay, here's a little back story. Last Summer my friend (male) invited me to go out for a movie/dinner with him and these two girls he knows. I didn't have anything to do and it sounded like a good time so I went. We all had a good time and I was really getting along with one of the girls in particular (and he clearly seemed interested in the other) so at the end of the night I told her that it would be nice if we could go out again some time and got her number. A couple days later I texted her, we made small talk for a bit and then I asked her if she wanted to go out sometime. She told me that she was "talking to another guy" at the time and that she didn't want to make things complicated. I told her I understood and moved on.

Well just a couple nights ago my friend invited me to go out with him and the two girls again for dinner and then a movie night at his place. I ended up going and we had a great time again. Except this time the girl (the one I was interested in) was flirting with me more than last time. It wasn't an arranged date, it was just us four hanging out for the night, but even my friend and the other girl were making jokes about how it looked like we were on a date together. We talked and had a good time and then ended up heading back to my friend's place for the movie. We had a blanket over us and she was laying on my shoulder. Needless to say I was confused because her behavior was definitely contradicting what she had said last time I saw her. The night ended without anything big really happening and we just dropped them off back at their place but it left me wondering if I should try and pursue the girl again. We had a good time together and we have a lot in common, but I'm not sure if she's just flirty or if she was really interested in me. What do you think? And if you do recommend trying to go after her, how should I contact her? I don't have her phone number anymore (deleted it awhile ago. I know, stupid) so would it be tacky to contact her on facebook or should I just get my friend to arrange another hangout night or what? I appreciate any advice, thanks. Oh, and I could see this girl being dating material in case you were wondering about my intentions.