There are quite a few. I know some Facebook projects are.

From the top of my mind https://github.com/styled-components/styled-components.

Here at Streak with have 10s of thousands of lines of code written in Flow. Our main app and the InboxSDK are not open source but we do have some smaller open source libraries on our github. Check them out!

https://github.com/StreakYC

Everything updated in 2017 (so from react-float-anchor and later) has flow types included.