It could be that the two of you spend so much time chatting on fb that you have nothing left to say to each other when you actually see each other. That's what I hate about social networking. It's takes the social aspect right out of the equation.
Talk to her. If she's your bestie you should be able to ask her if there's a problem. Guessing won't help you. Just sit her down, tell her how much you care about her, and tell her how you feel. Don't be confrontational, just be honest, but calm.
As for things best friends do together, I'm of a different generation, a generation that didn't have computers, or texting. A generation where we actually spent time with our friends, face to face, actually talking to each other, going out and having fun together.
Maybe you should arrange a sleepover. Put the cell phones away, don't go near the computer. Get a few chick flicks, make popcorn, and talk. Spend actual time with each other.
I wish you the best of luck, and I hope it all works out.