I've been involved in accessibility studies for almost three years and I have to say that Facebook is not exactly the perfect prototype of an accessible web platform. This is due to the following reasons:
- Proprietary markup used in APIs
Instead of using Microformats, Facebook's APIs rely on proprietary markup to make the application work. This leads to poor interoperability betwenn different user-agents.
Without a proper fallback mechanism, these two technologies can actually make life very hard for people with disabilities or people who use assistive technologies.
I hope that in a near future Facebook will take into account these issues more seriously. Meanwhile, people with disabilities may experience a genuine frustration in trying to use this platform.