Sharing the link to the interactive version from Photosynth.net also works for me.
Try going to your profile page, here on the site, loading one of your synths or panos (if you haven't uploaded any yet, just search the site for an interesting subject and choose someone else's upload to test sharing with - or use one of my uploads, if you like), then, when you are on the synth's or pano's view page, click the 'Facebook' link below the lower right edge of the viewer.
Clicking that link here on the site should result in that same ability to post a link to Facebook. That way, you can at least see what is meant to happen so that you have a baseline experience to compare your lack of good fortune with the app thus far with. If nothing else, it is a workaround.