Its an abbreviation for Call Me When You Get A Life. This can be used when you dont feel like typing out the whole phrase or you dont have enough space/characters left. Can be used on formspring.
Person one: you know what? you are a complete bitch. im not your friend, but i know a bitch when i see one.
Person two: Ok wow. If you dont even know her, why did you say that? She doesnt care what you think anyway. And im guessing that you don't have anything else to do all day besides sit behind your computer and write mean anonymous comments on formspring? cmwygal.