Hmm. Something has definitely broken there. Even before you click the [Share] button on Facebook, something is wrong, as the synth/pano's thumbnail is not being displayed, nor its title and description being fetched.
There's no trick to getting this to work. It may be that Facebook changed something and that the Photosynth team will need to update how they interact with Facebook, or there may be something wrong with the piece of the Photosynth site that handles this.
Regardless, sorry that this was your first experience. I'll make sure that the Photosynth team knows that this is a problem and hopefully it will be resolved for us soon.