The best advice I ever got was from my wife when she said "is what you are doing is useful for Facebook or are you just playing around?". I feel like this sums up my view of Open Source really well. First and foremost, I want to work on things that are useful and impactful for the people who are paying the bills. Then, if it's more widely useful, then open source it and invest time making it successful outside.
I feel like we've done a really good job so far navigating this, where we've spent 80% or more of the time providing value for Facebook with those projects and then with 20% make a thriving ecosystem around it that in turns benefits Facebook.