I think with online friendships in general, at least speaking platonically (and I believe this applies romantically, as I'll give an example in a second...) it takes years to truly know the person from talking to them online.
For example, one of my online friends, I met back in 2004 or 2005? I talked to him on AIM, and on the phone. Anyway, I finally met him in person last year. Oddly enough, since we both talked so much and stuff, and it was like....7-8 years of knowing each other, when we met in person, it was oddly seemless. Like I'd already known him in person. No like big surprises compared to online "persona" vs real life. But I knew him for a LONG time.
My friend ended up actually marrying a girl he met online. Great girl. He met her in 7th or 8th grade, on some RPG or anime board. They just talked to each other on AIM, and eventually started playing WoW together. She lived on the other side of the country, though. Eventually circumstances happened, and she moved closer to where he lived, in Brooklyn for acting school. And they met up in person. She eventually moved down here about a year later, and then maybe a year or two after that, they got married. With this, though, my friend met her when he was like 13 or something, and didn't end up meeting her until he was like 19 or 20 years old.
Another girl I was briefly acquainted with who worked at a store here, she and her boyfriend lived together, and her boyfriend and her knew each other for 3 years just online before really meeting each other.
Anyway, I think the biggest problem with an online relationship is it's still a distance relationship. Things take a lot longer. I'd not really consider anyone boyfriend and girlfriend or whatever just talking online. Because it's going to take a lot longer to know the person. Talking online, you're gonna see their good days, bad days, etc. If you do like this girl, then great. Go get a plane ticket this summer and meet up with her if you think she's really your "girlfriend." If not, be happy talking with her, get to know her more, and then time will tell you everything you need to know.