How to ask a shy guy to prom?

So there is this guy and he and I have been talking recently. He is very shy and we don't know each other that well. I was wondering if it would be weird or awkward if I asked him to go to prom with me? Should I just go up and ask him in person or since we don't know each other that well over Facebook, or is Facebook just tacky?
How to ask a shy guy to prom?
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