Facebook's competitive advantage doesn't come from having the best reliable streaming data store at scale, or from its software in general. Even if MySpace, Friendster or Google + got their hands on the whole software stack & started running it, people would stick with Facebook.

So there's no cost to open sourcing. The benefit comes from being known as technically innovative in general, and for recruiting, being known as having interesting, meaty, challenging projects to work on.

The impetus usually comes from team members who want to do the work. It could be to become known for having worked on the project, or a sense of giving back to the community, or a hope that you'll get bug fixes & features from outside contributors. In my (very limited) experience, managers "passively encourage" it -- they generally don't push the team to do it, but when the team asks, they encourage it.

Look at React - if it had never been open sourced, Facebook might still be using it, but it wouldn't be the same thing it is today. For one, basically all of the current excellent React team probably wouldn't be working at Facebook. And it would be far, far harder for Facebook to recruit engineers for product teams who were proficient in React. Since it is open source and very popular, the odds of a browser introducing a change that unfixably hurts React's performance is now very low. Et cetera.

The cost of maintaining an open source project is real, but when it is a world-class piece of infrastructure, open sourcing it helps keep it world-class.

Branding / Marekting / and more importantly, attract talent to work for them.

Institutional dependency?