> Could they not have upstreamed those features in the first place?

This can be harder than you think. Some time ago I worked a $BIGCORP and internally we used an open source library with some modifications to allow it to fit better into our architecture. In order to get things upstreamed we had to become official contributors AND lobby to get everyone involved to see the usefulness of what we were trying to do. This took a lot of back-and-forth and rethinking the design to make it less specific to OUR needs and more generally applicable to everyone. It's a process. I'm not surprised that Facebook's initial approach would be an internal fork instead of trying to play the political games necessary to get everything upstreamed right off the bat. That's exactly the situation we were in, so I get it.