This didn't originate from within Facebook. It is an evolution of the work done in late 2013/early 2014 around FSE[1]. Facebook later hired Yann to support his work around this (pretty amazing!) tool.

The ability to define your own dictionary is useful beyond simply large indexable datasets. Anything application-specific can usually benefit from user-defined dictionaries.

[1] http://fastcompression.blogspot.com/2013/12/finite-state-ent...