Facebook recognises Zawgyi-One as a separate language used in Myanmar. This is the result of the most Burmese speaking/typing users using Zawgyi-One font.
According to Facebook, Unicode is denoted by
my_MM and Zawgyi by
qz_MM. They label it as Burmese (Zawgyi).
MM is two-letters country/region code for Myanmar and my is two-letters language code for Myanmar (Burmese). They use qz for Zawgyi-One font. According to Best Current Practice for Language Tags by IETF, QZ is reserved for private use.
Myanmar unicode is superior in technology but adoption rate is low according to various reasons. Zawgyi-One is not unicode standard compliant and no longer being maintained or developed. However users are familiar with it and it is going to stay for a foreseeable future.