The only other alternative, really, would be to have Bing Images examine every last image on Facebook for image features and always be crunching away trying to match the image features on the Bing servers. This isn't necessarily so far fetched, being that Facebook seems quite happy with Bing (it's their default web search engine, default mapping platform for Facebook Places, etc. etc.), but this would certainly be a large project.
I'm also certain that not being given photos which have been chosen by a human because they all belong to the same environment (as is the case when people make synths intentionally) would complicate making reliable matches between images. What would help is if you could geotag individual photos or entire albums at a time. If an entire album was shot at the same location, those could easily be cherry-picked for potential synthing, but I would be careful to have manual photo alignment ready for the Facebook audience.