Newer versions of iPhoto have a "Facebook" button (and some other social networking/photo sharing links) in the lower right-hand corner of the application window.
If you don't see those there, from the "View" menu, select "Show in Toolbar > Facebook".
If you don't see these options or the menu layout is different, you may have an earlier version of iPhoto that doesn't have this capability. In this case, you can simply drag photos from iPhoto to your desktop, then log on to Facebook via a web browser and upload photos from within Facebook.