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.
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.
Tom: I have no clue who this is.
Tom: Crap, I have a case of Facebook Alzheimer's