I've been reading an interesting blog on relationships and life in general and one of the most paradigm shifting gems I've found is, nothing and no one is extraordinary if you see them as such, you have invested emotions in something completely unknown. Think of it this way, if you've had girlfriends you've loved before, there must have been a point when you thought they were unique, special, extraordinary, perfect, irreplacable, but then it fell apart and you found someone new and you thought exadtly the same of them. It's the same with crushes, you admire them, you feel lucky for having a chance to maybe get close with them, and you're psyching yourself up on what an amazing opportunity you've got (especially in your case when you regret not doing something) and you don't want to mess up, because she's so pretty. I'm still learning to practice this kind of positive indifference, acceptance, make a step, but let the chips fall as they may and you should be in a state that if you get a NO, you're not down and depressed and want to shoot yourself because you're sure you messed up or missed a shot, and if you get a YES, you shouldn't feel extatic and overflowing with joy. A no should go down like - ok, no, (shrug), cool - and move on. A yes should go down like - ok, yes, cool - and see where it goes. Remember that there are no certain things, a girl that initially rejects you, may get into you later, and a girl that accepts your invite might turn out to be a really bad choice and if she's not, and everything is great and amazing you still shouldn't succumb to extasy. In order to do this, though, you need a stability in your life and have to be self-sufficient, if you see a girl as a means to push you a step closer to happiness you will get hurt every time.
So, what to do? You added her - this is important, she still may be interested in you, but you can't know. If she's not that into you, it is super easy to reject you over FB, and she won't wast a second about it. If at all possible, wait until you meet again. I wouldn't invite her to a date, it's just too impersonal (in old timey terms, you met her, got home, opened up a phone book found her number and now you wanna call her to ask her out), if there's a thing you're gonna do with your friends invite her there and then see how things go.