Facebook uses EXIF data for descriptions

Facebook extracts a photo’s EXIF and IPTC information and uses the title and copyright information to fill in the description for the photo.

You can add this data in Adobe Photoshop, Bridge, or Lightroom and Facebook will place it in the description even if someone else uploads the file.

Note: This only works the first time the image is uploaded, once it is on Facebook, all metadata is removed from the image.