Hate to tell you this Smootha, but love at 15 is definitely different than love at 19. Not to be mean or anything, but get someone closer to your own age and leave the child alone. (I guess I feel so strongly because I got married at 15, but was divorced at 19 - that "love" I felt so strongly changed as I matured.)
This is a very real answer: Put her well-being first, let her go. She is too young to be tied to an older guy who lives a long way away. Make a plan to get together again in two years, it'll work out if it's meant to be.
You arent really in a relationship. You guys have never even met. If you want to stay in your facebook love thing you got going on... you will have to do it for about 3 more years for her to be legal. Otherwise, you can get in trouble.
I have been in this situation, more than once, and have only just recently gotten out of it. (not my choice) It does work, it does, if the love is strong enough. I can't tell you how, just keep doing what you've been doing. All love, not just online, fades to that comfort level, once the "in love" phase has disappeared, and it always does, given enough time together. Just tell yourself that one day you two will meet. If you really need to do that, though, maybe the love has faded too much already. Good luck, and it really can work, but I know the physical longing and emotional longing can really be hard to deal with.
Listen Smooth, be very careful what you talk about with this 15 year old. You can end up on a sex offender registry for things you probably feel innocent. All it takes is an angry parent or jealous wanna be boyfriend. Best advice, check legalities for each state you live in. Besides that, isn't she a little young.
It an can work, it's going to be hard for both. Be careful what you say and how you say things. Be prepare for if she ask you to see other people and be in a cyber relationship with you, don't get your hopes to high. Good luck.