Comment by danso

FWIW, Facebook's head of open source, James Pearce, had an excellent interview with Changelog about FB's motivations and challenges for maintaining open source projects: https://changelog.com/podcast/211

According to Pearce, the amount of resources to manage open-source is non-trivial. For now, there's a decision process before deciding whether a project should go open source because of the maintenance cost and because they don't want dead projects under their banner (e.g. codebases that don't get used by the non-FB open-source community). One interesting point Pearce brought up was that React takes community contributions, but FB's policy is to have a single version of React, i.e. the React that's released to the public is exactly what runs on FB production, which requires a certain level of logistics.

The main benefit of OS, besides the free labor from the open source community, is public adoption of the software, which gives FB some kind of leverage in the software world. But it also means that job candidates can come in already experienced in React etc.