A lot of big tech companies also use Yahoo chat. Salesforce did as late as 2012 when I left, with 1k+ people in the eng org. (No idea if they still do).
Doubtful; Yahoo stopped supporting OS X in the last couple years with the official YIM client, and the service is completely unreliable on the platform with 3rd party clients, with messages randomly failing to be delivered. Pretty much any company with non-Windows users has been forced to migrate away, including my workplace (thank Jebus).
Seemed crazy at the same, still does now too