You suffer from this disorder when you don't remember a person, who is trying to add you as a friend on Facebook, and they remember you from a past school or job.
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.
Monica: Hey Ross, I don't remember Charlie from high school at all.
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.