Alt text allows users who are blind or visually impaired to experience imagery because screen readers can use them to describe the content.
Unfortunately lots of images are posted without alt text. In 2016, Facebook introduced a new software called automatic alternative text (AAT). This used object recognition to describe images but needed improvement over time. The latest version has 10 times the reliability and can now identify activities, landmarks and animal types. It has also been adapted to understand relative position and size of objects in an image. This way it can identify how people are standing, or what the main focus of the image is.