Combination of FOCL (Falling off chair laughing), ROFL (Rolling on the floor laughing) and LMAO Laughing my ass off). FOCROFLMAO therefore is "Falls off chair rolling on the floor laughing my ass off".
As a reaction to a funny comment or joke told over the internet.
E.g. Joke Teller: ...so the guy replies, I didn't know your dad was a pharmacist.
Listener: FOCROFLMAO!!!!!
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.