Two YCombinator projects are using Rails and React
Work at a Startup and
Bookface, two YCombinator
projects are using Rails + React + PostgreSQL

Source:
- https://www.ycombinator.com/careers?ashby_jid=00c6950f-341f-4924-a456-ea32c9d5601d
- https://www.ycombinator.com/careers?ashby_jid=ef00c8d1-76e7-4cc3-82fe-7cc3e4679652
Update:
Here is a video from YCombinator where they explain the
choice for Rails:
https://youtu.be/BJjsfNO5JTo?t=801