Talk to girl on facebook or not?

Talk to girl on facebook or not?
Ok well im a senior in highschool and less then 2 weeks left till graduation.
I really like this junior girl and I know she likes me back.
I mostly see her in the school hallways or parking lot.
Never talked to her before and when i get close to her she gives me a little smile kinda.. so, I am shy and since there is no way to just walk up to her in front of all the people in school and say hi with no reason, i though i should just talk to her on facebook. Is that ok?
WHAT SHOULD write to her on facebook?