Facebook has had the accesibility feature of describing the photos on your stream (automatic alternate text) avaliable for some time on their mobile applications.
https://www.facebook.com/accessibility/videos/1082033931840331
This should be coming to various platforms soon but if you cant wait, and want to catch a glimps on computer vision advancements within Facebook, here is a userscript for you:
https://greasyfork.org/en/scripts/22304-facebook-image-alt-text-display