How can I talk to this girl on Facebook I barely know?

I have her as a friend on Facebook, but we never really spoke to each other. I just kind of know her from high school and basically everyone added each other when they were in the same class. I'm in college now and she's also there but not in my class.
So I feel like I don't really have anything to say after I say "Hey".
She does like some pictures of me, does that mean anything?

Thanks
How can I talk to this girl on Facebook I barely know?
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