Martin Howard , Jun 04, 2010; 08:29 a.m.
To avoid confusion you should think in terms of number of pixels when resizing images for the web.
Facebook supports a maximum image size of 720 x 720 pixels, so assuming your shots are 3:2 aspect ratio then resizing them to 720 x 480 should do the trick. You could probably still get an OK 6"x4" print from this, but that's about all.
Note facebook recently increased the upload size to 720 from 604 pixels, there is nothing to stop you using a smaller image size of course.