Friendnesia is when you don't remember or recognize the person who has sent you a friend request on Facebook.
Jack: "Hey Jill, do you know who Little Boy Blue is?"
Jill: "No, Jack. I don't think I've ever heard of him. Why?"
Jack: "Well, he just sent me a friend request and I don't remember him or recognize him from his picture."
Jill: "Ah, a case of friendnesia, eh? I get that sometimes too."