I'm not sure I'd call it an 'ulterior motive' as its so obvious but there's obviously a massive benefit to Facebook from this - the potential unconnected billions will all get Facebook accounts and probably be grateful to Facebook for providing free internet. It gives Facebook a way to go from 1bn users to 5bn. I don't know if it's that much of a problem if I couldn't get net access otherwise then having a device with Facebook on it but not say Diaspora would not worry me hugely. They can always get regular open internet access when they have some money together.