A mobile device is most-often used by the subject of the photo and held at a downward angle. This eliminates any chin and also shows cleavage - both are indicative of girlfriend lips.
The exact purposes for this pose remain unknown, however, among the various motivations: to appear slutty, to attract the lowest common denominator of male, to make oneself appear more thin than is actually the case.
All of the chicks on my Facebook do the girlfriend lips in their picture. Why don't I ever see them do that in real life.