How do i get a girl to message me back on Facebook?

[ColdRush123]

So i messaged a girl that i don't really know a simple hello with a smiley face and its been 3 days and she still hasnt message me back. I know she has seen it because it says she's seen it. And this isn't the first time it has happen to me. So how do i get her or some other girl to reply back to me.

[PandaTrainer]

Ask them why they decided to add you but won't even talk to you.

[idk]

This isn't the right section for this. This is more relationship stuff. Did you try telling them they are pretty like a princess? Or how about saying "knock knock"?