This is an old bug where the camera coordinates on Photosynth 1 'Share Link' URLs are longer than Facebook's maximum character limit for URLs.
Because of this, Facebook is inadvertently chopping off the last half of the camera coordinates and you end up loading into the synth or the pano at unexpected locations or orientations.
There are two ways to avoid this scenario.
1) Unless you want to link someone to a specific zoom level or a different initial direction than the default, don't use a Share Link. Instead, just use the view link from your profile.
2) If you definitely do want to use a Share Link for its intended purpose of sharing a specific first view, you can use a URL shortener like http://bit.ly which will give you a short enough URL for Facebook to leave alone, which will redirect your users to the full Share Link when clicked.