As a frame of reference, my single core AThlon 64/3500+ shows only 2-3% useage when at FB; perhaps you have some goofy Farmville or Fishville app running?
I asked if they're running FireFox because as much as everyone seems to love it, there is (or was) a huge bug where it randomly corrupts itself and runs at 100% after a few minutes of use. Only way to fix is reinstall. Facebook - even as a large and constantly refreshing website - doesn't (or shouldn't) use any more CPU than a normal website.
Give us a bit more information grandmasjoys and we'll be able to help.