The friends or family of your ex you don't know who you have accumulated through relationships from your on Facebook.
Friend: "yo who is this girl"
Me: "she was best friends with my ex never met her though now she is one of my leftover friends"
Leftover friends are friends of friends in school which are now your main friends due to the fact that your original 'main' friends have moved out of the area and left you with the....leftover friends. Can also apply to someone introduced at work by a friend which is now a better friend than the original.
Now that my friend Jill went off to college, I am stuck with my leftover friends she introduced me to.