Since April 2016, Facebook has been automatically adding alt tags to images you upload that are populated with keywords representing the content of your images.
This Chrome Extension visualizes the tags (which can be found in the markup)
<img src="https://facebook.com/some-url.jpg"
alt="Image may contain: golf, grass, outdoor and nature"
width="316" height="237">Show Facebook Computer Vision Tags Source (GitHub) →
Show Facebook Computer Vision Tags on the Chrome Webstore →