What is the hexadecimal code of the "blue" background color of Facebook?

The colour of the main blue banner is #3b5999, and the lighter blue colour used for the backgrounds of comments is #eeeff4. A way to figure this stuff out is grab a screenshot and open it in image editing software that has a colour picker tool (e.g GIMP)

You can also use firebug, for firefox and it can show you the css attached to any element with the inspect tool.

Look at here: http://webdesign.about.com/od/colorpalettes/ig/Color-Palettes/facebookpalette.htm

Colors are:

#0x3b5998
#0x8b9dc3
#0xf7f7f7
#0xdfe3ee

the blue color used by facebook:

#3B5998