Typically speaking, you two are bound to have at least a few mutual friends as the only evidence on Facebook that you know each other.
The only known cure for Facebook Alzheimer's is to look up the person in a past school yearbook as a way to jog the memory that is encapsulated somewhere in the hippocampus. It best to do an yearbook lookup reference ASAP, before the forgetfulness gets any worse.
Ross: Hey Mon, he's like the only guy who noticed you when you were fat.
Monica: Well, now I'm hot, all guys notice me.
Ross: Well, I would have to say that you are suffering from a disorder syndrome known none other than Facebook Alzheimer's.
Monica: O no, I got to get my yearbooks out of my closet and try to jog my memory, as soon as possible.
Basically, it is somebody who does not remember you, but you remember them and you want to be facebook buddies.
Chandler: Well, how do you remember her?
Joey: I like slept with her three times.
Chandler: She probably has Facebook Alzheimer's.
Joey: How come girls suffer from Facebook Alzheimer's more often than guys.
Chandler: It's because of lower brain capacity in females; well, you see, when girls go off to college, they have to make a choice - either remember past sex partners like you, or gain further knowledge in their college major.
Joey: Very intriguing.
Tom: I have no clue who this is.
Tom: Crap, I have a case of Facebook Alzheimer's