Also, I think that in reality there isn't as much brand loyalty as people think. Three years ago pundits said the same thing about myspace.
My position is that facebook is vastly overvalued - Gartner was just an example to show that even if they become the biggest in the field the income would still not justify their evaluation.
In fact Gartner has a market capitalization of 1.3 billion US$, meaning that facebook should be worth more than 12 times gartner. And Gartner has 4000 employees, 60.000 clients, and offices worldwide.