FYI: I've written a workaround for the Facebook "preview" when posting links to Garmin Connect activities, following the pattern of MapMyRun. As long as your activity is set to public, you can use this link: http://randywestergren.com/garmin/index.php?activity=ACTIVITY_ID where ACTIVITY_ID is the numeric value in the URL of the Garmin Connect activity details page. When a user visits the page, they'll be redirected to the actual Garmin Connect details page.
Before: http://i.imgur.com/1MeFNhJ.png
After: http://i.imgur.com/hKZ5vTG.png
