Do they have a specific PHP-to-Node bridge on the server side? If they write isomorphic code, either they are writing apps completely separate from PHP or they have some kind of integration (Node-in-PHP?) running?
I would be grateful for hints, I'm looking into working more with FB tech but I can't do Node on the server right now. Knowing how their architecture looks like with PHP/Hack on the backend would really help.
Relay has a special server rendering mode that we created that I'm hoping to speak about soon. Until then I'm afraid I can't say more than that :)
For server side rendering of the React app, they use https://github.com/reactjs/react-php-v8js (well they probably use a slightly customized version but this is what they open sourced).