Sometimes it's hard for "spare time" and "work time" to separate (see burnout above) since people are often just working on the things they're excited about all the time.
For me personally, since I work on our Product Infrastructure team, my main priority is building the right tools and libraries for Facebook's projects. I do far less direct product work, however I still do a little.
However, I used to work primarily on product, but still spent some of my time on product infrastructure. GraphQL was originally born while I was working with the Mobile team on redesigning News Feed. At that time I was spending part of my time designing News Feed, and part of my time designing GraphQL, and the two became fairly inseparable as getting GraphQL at high quality was critical for getting News Feed at high quality.
Open Source is a little different since a lot of the primary audience is outside of Facebook. I've found that the best way to prioritize it is to ensure that it aligns directly with Facebook's goals.
Only open source stuff that's important to your company. That ensures that continuing to work on the things that are important to your company benefits open source, and open source contributions are far more likely to benefit your company.