Would it be weird to randomly message a girl, and, if not,what should I say to her?

[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

[Kayla]

Don't be afraid to talk to her. You'll get nowhere if you don't. If you notice she's online, just say "Hey, what's up?" Learn to small talk.

[desertcities]

Nothing ventured, nothing gained. I say go for it. You can both FB 'Poke' her (a bit strange I think) and just write her a short message. Ask her what does she like to do after church usually? Or, since she missed a service, share with her what she missed. Something she might have liked to known.

Good Luck!

[Joseph]

Normally, I wouldn't suggest a random Facebook message, but this is a bit different.

Send the message, preferably when she is on Facebook, just saying "I was waiting for today (Sunday) to wish you a happy Valentine's Day, but you weren't in church. I was a bit worried when I didn't see you and hope that you aren't sick."

If you know that you'll be seeing her in a few days, wait until then. I still really think that in person is best. Actually, when you see her, you could bring her one rose (pink, not red) or one of whatever her favorite flower is and wish her a belated Valentine's Day. This should get the conversation going. Also, this would be a good time to ask for her number.

Good luck!