I think both points are true: FB does want to be the Internet, but they also realize that the path to that endgame lies in capturing (and driving) behavior, not

I think both points are true: FB does want to be the Internet, but they also realize that the path to that endgame lies in capturing (and driving) behavior, not in building infrastructure. The latter is just a necessary tool. But, you're not going to threaten FB 's mission by simply building out an identical (or even superior) infrastructure. So, there's virtually zero-risk to them in open-sourcing it.