I don't want to repay my ex the money that I owe her.
Is there some reason that you don't want to repay her?
She's blackmailing me to be her friend on facebook.
I understand that your ex is blackmailing you, and that is the reason that you should pay her back. You should always pay people what you owe them. Then there will be nothing for a person to hold over you.
That's a really good point. If I pay her back she'll probably leave me alone.
That's exactly what I'm trying to say. If you give her the money she's asking for, then she'll have no reason for her to keep contacting you. I think it would be better for both of you if you gave her the money.