So, this is girl added me on Facebook.. And I'd say she's about 9, but she gets a lot of attention from guys so I was thinking of dropping a challenging opener instead of something she probably gets commonly.

I was gonna go with "And you are? lol" or "are you gonna introduce yourself or what? lol"

give me feedback or feel free to give me other ones.

make sure to not piss her off though cause if she gets a lot of attention from guys then shes got the choice of rejecting you which means she has the power. try saying something like "hey you know its not polite to request strangers and then not introduce yourself " that way you challenge her a bit but youre not gonna go to extreme and piss her off. you need to flip the roles in your conversation later on to make it that YOU have the choice which gives you the power which gives you the attraction. girls love guys with power. but if i were to choose yours id go for the second one
choice=power=attraction

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"Ever notice how you come across somebody once in a while you shouldn’t have fu***d with? That’s me." — Clint Eastwood

I'd say that's a safe way to go. Keep it playful definitely. I'm not sure if you should send it as a chat or as a message. I think chat would work best. I started a whole relationship once by talking to a girl on fb even though I only spoke to her for like 10 minutes cumulatively before that. I was just in the zone and I nailed cocky funny. Read up on cocky/funny and apply that to this situation. But yeah, I think you've got a decent framework for starting, read up on it though before proceeding.