I saw no mention of Reason or ReasonReact. I thought they rewrote Messenger in Reason, and so I figured facebook.com would be next. Did Reason fall out of favor or something?
The rebuilding of facebook.com was done quickly and involves almost every web team at Facebook. Almost all of these folks are familiar with Flow and only some are familiar with Reason. Asking every engineer at FB to learn Reason at the same time in addition to the already massive number of new things would have been adding unnecessary risk to this already incredibly risky project.
https://github.com/reasonml/reason-react/blob/master/HISTORY...