React, Flux, GraphQL, Hack, HHVM...? All of this and more!
Despite it's size and focus at the consumer market, Facebook is very much a company driven by software and developers. This shows in their efforts of creating (sometimes rather unique) solutions to their own internal problems and then sharing those as Open Source. Just like the Symfony community, Facebook invests in Developer Experience by providing development tooling for these solutions.
This version of the developer tools is written completely from the ground up, supports Firefox, Chrome and other Blink browsers. It is also a fine example of what is known as Dog Fooding in the Software industry. React developer tools themselves are built with React.
See animation below for React devtools in action: