Earlier quoted context omitted.
The amount of data Facebook is able to collect on their users is the difference for them, and the value of data for hundreds of millions of users is hard to quantify.
facebook's core offering is slowly walking towards the exit: http://www.google.com/trends/explore#q=%2Fm%2F02y1vz . Much of that valuable data stands to stale. (edit: how do i link.. oh man)
That seemed pretty convincing, but then I added some comparison terms like "instagram" and "snapchat"... they are hardly a blip. I think that argues that the world is going mobile, but then maybe google trends isn't a fair way to measure who is walking where.