How to ask a girl for her Facebook through text without sounding creepy?

[Sonnan]

I met this girl and an SAT Prep. Class and we saw and talked to each other every class (which was 3 times a week) on the last day of the class we exchanged phone numbers. Now the first time I talked to her through text was the day before the SAT and wished her good luck. After the SAT we started talking. Next time we text each other was because I wished everyone on my contacts "Happy Summer Break" and she replied and so we started talking about plans for the summer. I let her talk for the most of it on how she just visited NY and camps she's going to. Now I have ended this conversation with her and I just want to ask her for her FB but I do not want to sound creepy nor do I really want it to come out of the blue. So how do I start a conversation with her and then ask her for her name on Facebook. Let me say that I do not know her last name so I can't really add her without asking. Please help, and Thank you