Would it be weird for me to randomly message a girl?

[Charlie]

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:
Today's Sunday, but she wasn't at church today 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?
As well, as valentines day was two days ago, do you think she may be a bit suspicious

[Bringing Salvation]

well she liked your status you could message her about that use tactics dude lol

say hey how are you noticed you liked my status XD
how are you
banter back and forth then fish for info you want to know but dont jump right into the topic.

or something on those lines play it off take a baby step at a time dont rush in all hell blazing.
but be yourself and go with flow.

[abraXus]

talk to her in person about being friends on facebook first