We use Immutable.js in a lot of our client code, especially along side React and using an architecture where there is a single immutable state at the top of the app. Our Ads Manager uses Immutable.js for its apps on the web, and in their react native based iOS and Android apps. Our Messenger web (messenger.com) uses it, and commenting on anything in news feed, or writing a new post uses it in the Draft.js project (https://facebook.github.io/draft-js/).