Before facebook, people thought their email was their online identity, they had all their friend and relatives in their contacts, why bother register on facebook to recreate all that?

As the consumers become more tech-savvy (old dogs can learn new tricks) and technologies become easier to use, it will be easier for them to switch service. All it takes is the influential people to move (think celebrities, popular friend, active family member, etc) for the others to follow.

Yes of course it is possible and will get easier to transfer services. Question is will the 500MM users do it en-masse? FB would have to screw up pretty big for that to happen in the next 3-5 years, in which time, it MAY be possible to build an enormous multi-billion dollar (or even deca-billion $$) per year profit stream.