I came upon this problem when I wanted to edit/apply an exif image description and have that recognized by Facebook.
As it has been mentioned FB does strip some exif data.
I used to use the linux version of Google Picasa and the image description would be preserved upon FB upload. However as Picasa on linux has finished.
I have now used Phatch to do more advanced exif editing and writing. However its quite advanced and will not tell you what exif tag to write so it gets preserved for FB.
I found at least two tags that would possibly apply. One is an exif tag
and another tag whitch is actually an IPTC tag
It turns out that only the <Iptc_Application2_Caption> tag was recognized and actually ended up in the Facebook image description.
This is my small contribution on the matter in case somebody has the same problem. I do understand the matter is more extensive and I would even dream about claiming to know the whole story.