Would it be weird if I randomly messaged a girl?

[Charlie]

4
Basically, there is this girl that really like from my church. I usually see her once or twice a week and we talk a bit - not lots though. I have never chatted to her on Facebook before however I really want to, to be honest. She liked my latest status update.. Would it be weird for me to randomly message her and, if not, what should I say? I'm not the best at starting conversations. Should I only msg her when she is online?
Thankyou in advance

Extra information:
She wasn't at church yesterday so I'm guessing she is on holiday.
I think she likes me a bit because she looks at me differently to other girls, we both have the same favourite animal, she knows I love guitar and she plays flute, I'm a year older and I wouldn't call myself popular at school, I am, I think, a bit more at church though. I don't have her number atm , how could I get that because is it weird to ask her this late?