Is there somewhere that you have the option of sharing records? Unless you're seeing something that I've never seen, I don't believe that you can share the records themselves on Facebook. You can, however, share the activity that you set the record in. If you go to the activity details page, click the lock icon and select Everyone, then click the share button and click Facebook. If you want to tell everyone that this was your 5K PR, then add that in the comment, then click Share.
