They benefit from other developers using it or they wouldn’t go through the trouble of making it accessible outside of the company.
Access to good talent runs try. Maybe you grow a team of highly talented developers using Facebook’s stack. Facebook is more capable of recruiting them since they’re already familiar with some of Facebook’s stack.
There are interesting reasons why companies expend a lot of resources on free stuff for programmers. If it didn't have a beneficial effect, they wouldn't do it.
Access to good talent runs try. Maybe you grow a team of highly talented developers using Facebook’s stack. Facebook is more capable of recruiting them since they’re already familiar with some of Facebook’s stack.