It's because MicroB(the default browser) faithfully follows the HTTP standard. Opera does too, but it uses special compression to reduce the data usage.

What am I talking about?
Apparently, Facebook has a... problem. Whoever designed/works on the site majorly screwed things up; The headers of *all* files tell the browser to re-download it every couple of minutes, expecting changes. This results in ridiculous amounts of data usage(and wasted power), even when it doesn't seem to be doing anything.
A properly configured site would have static content -- Like images - cached for several days at least, and dynamic content -- the small amount of HTML data -- reloaded fairly frequently.

At least, that's what I heard on Slashdot. I *refuse* to even touch the site with a 10-foot poll myself, so I can't really check myself.