I'm going to message an old crush on Facebook
Do you think it will end up in a date?
We'll see if I'll get a positive or a negative answer, but judging from her past behavior I'm not expecting much from her.
If you already think she will not respond, then maybe you should not message her. There are other people to talk to insead.
What is the harm in trying?
You said yourself that she will probably say no. Then you should take the time to talk to another girl with a higher chance of saying yes.