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... after about 12 hours.
Joscelin, have you tried loading it in a different browser or a private window (anything that would avoid loading the file from your browser's cache of previously downloaded files)?
I have now. The thumbnail has been updated quickly in Photosynth. No updates in the facebook web app on different browsers, app on Windows 8.1 and app on WP8.
Thanks for checking.
Now that you mention it, I have had Facebook cache a thumbnail for a link before and refuse to update it, even when I'd updated what it was pointing at (variations on linking to a YouTube video, if I recall correctly), so it may be that Facebook is caching the metadata and not fetching it fresh from the source site.
If true, I'll admit that it makes sense for Facebook, being that millions of people may share the same link and it makes more sense to check their link database first to see if they've already got a thumbnail and summary than to fetch it over and over again, but it's less than helpful for those of us who wish to edit something and expect an updated preview.
I would think that a video preview of a synth is something too large for Facebook to want to cache, but I'd be curious to open Fiddler http://www.telerik.com/fiddler and verify that the video preview you're seeing on Facebook is indeed being served from Photosynth's servers.
Reading over this again, I'm realizing you probably just mean the JPG thumbnail before you actually play the video. Is that correct?
If so, from what I've read on the web in about five minutes, you should be able to force Facebook to re-fetch any shared link's metadata (summary, title, thumbnail) at this link: https://developers.facebook.com/tools/debug
More search results on the subject: http://bit.ly/udfbcache
Yes, I am referring to the jpg thumbnail. That influences the audience to click it or not.
But, the funny thing is that I could see an updated thumbnail in my Photosynth gallery, clicking for Facebook share produces the interface dialog with the original thumbnail nevertheless. May be something is going wrong there.
Just to be clear, Facebook's 'Post to Facebook' dialogue for sharing links is using Facebook's own database of information about links.
Sorry if I didn't make this clear before, but the way it works is that because Facebook has so many active users and many of them will share the same URL, Facebook only fetches a thumbnail for a page's URL the first time that someone puts that link in Facebook's database.
In your case, it fetched it the first time that you shared the synth.
Then when you saw the thumbnail, edited the synth to have a more desirable thumbnail, and reshared the link, Facebook just pulled up the thumbnail that it already stored for that particular URL.
The only way to get Facebook to fetch an updated thumbnail for that exact link is to open that debug page, erase Facebook's information about that link (previously fetched title, thumbnail, text summary, etc.), and then share that link again, which will allow Facebook to get an updated thumbnail etc.