It's probably simplest to delete your existing desktop shortcut to Facebook (via right-clicking on it and selecting 'Delete') and creating a new one.
To do so, go to UKB's link. Make your browser window small than full screen (by clicking on the overlapping squares at the top-right-hand corner). Click on the small icon immediately to the left of where the web address is displayed (which, since it's a secure site, is likely to be a padlock symbol) and drag it onto your desktop. Job done!