I've found some old posts about this issue, it might be you have a quick solution at hand
It's about the ImageLinkHover (version 1.0 - December 2011) that came with the Clarity Template. I also would like to be able to use External URL's linking to Facbook & Instragram. Currently I'm only able to link to menu items. I've tried to link to a menu item with an external URL in it, didn't work...
Any help would be welkom!