A "crack" is another word for a joke. It seems like a strange choice there, though, because the vaguely amusing line comes after the word, not before it.
Note that you have some errant quotation marks in your post. There should be opening marks before "Now" and closing after "time" and that's it for the first sentence. I don't know why you've got "what Facebook is" looking as if it's not part of the quote. It is.