Three other (somewhat related) reasons I’ve heard stated by FB folks:
1. Makes it easier to identify talent out in the world that they want to go after.
2. Raises the technical reputation of Facebook, making it easier to get candidates to say yes.
3. Makes it easier to ramp new employees up on Facebook’s stack if they are already familiar with a lot of it from open source.
Other times it's lock in for related technologies (current or future plans).
Some have speculated that it's a legal trick, like the problem everyone was insinuating with the React license before the recent change.
Anyone else have reasons I'm missing?