Please note that Facebook will disregard virtually all metadata you have applied to your images, including keywords and captions.
If you use Facebook's uploader to create an album, then it does indeed retain the caption and uses it as the photo's description in a post. PM does not have a Facebook uploader, but actually a lot of the uploaders out there that do exist don't make it easy to tag other people in the post. So, even if PM did have a FB uploader, it would still probably be more effective to use FB's native uploader - since there you can actually tag people and pages in photos. Plus, here is a tip that is not exactly intuitive: (This works on Chrome, I don't know about other browsers.) If you click "Create Photo Album" on Facebook, a file chooser dialog opens up. If you drag a selection of photos right from Photo Mechanic to the file chooser dialog and drop them there, the file chooser goes to that folder with your photos already selected. Upload them right there, and your caption fields will be the description for each photo in the album.
(Clarification: If you use FB to create a regular single-photo post, the metadata does not show up. Only when creating an album, does the caption come through, for whatever reason)
I hope that helps.
-Mick
