I personally feel that in this case the improved code can be more readable and doesn't need any other libraries by using full es6 potential
nodes
.map(node => node.followers.map(user => user.email)) // get all emails in a format [['abc@abc.com', 'ijk@ijk.com'...], ['abc@abc.com', ...], ...]
.reduce((emails, result) => [...emails, ...result], []) // flatMap like function, flat all the arrays into one
.filter((email, i, self) => self.indexOf(email) === i) // get all uniq items
.filter(mail => mail) // remove undefined or null