Something to try quickly to see if there are some Facebook app permissions problems:
1) In the Photosynth app, verify that you're in a "not connected" state for Facebook in the Settings tab.
2) Go to your Facebook Account Settings through http://facebook.com and remove Photosynth app permissions ( Account Settings -> Apps -> Photosynth -> remove)
3) Open the Photosynth app and connect your Facebook account in the Settings page by tapping on the "not connected" text. Follow the prompts making sure to authorize the app to access your Facebook account.
4) Try to Share your panorama through Facebook again.
Note, there have been a couple of cases where the initial authorization is unsuccessful and you see the error you received. If that happens, try to connect again through the Settings page, or just by Sharing a panorama.
Gabriel - Photosynth