Flake: an unreliable person.
Someone agrees to do something but never follows through. Such a person would cancel appointments repeatedly, often not giving any reason or explanation. A flake will e-mail, text or call you constantly to make plans but will not suddenly call it off at the last minute without explanation. Have you ever flaked out on a friend?
Read this article on how technology-aided communication encourages flakes: http://www.huffingtonpost.com/rachel-ryan/flake_b_2617464.html