GarminConnectAddThis is a project mainly written in ..., it's free.
Garmin Connect's integration with AddThis for Facebook is shit. I'm attempting to see whether I can quickly knock something up to make it a little better than shit.
Quite frankly, the Garmin Connect / Facebook interface for adding stuff to Facebook through AddThis is total wankery, because it doesn't even display an image. I have no idea how AddThis works at all, so I'm setting this shit up to try to figure that out, and then to see whether I can easily make adding my runs to Facebook a bit better than using Garmin Connect's interface.
Another option is to use MapMyRun's web site. But it's also fucking terrible.