Why is Diaspora built on Ruby on Rails instead of Node.js?
My answer to Why is Diaspora built on Ruby on Rails instead of Node.js? on Quora
Because Node.js had almost no visibility at all six months ago when Diaspora started. Also, Node.js has only very recently stopped breaking API backwards compatibility on a regular basis. Plus the Ruby library ecosystem is much, much larger than the Node.js ecosystem.
I love Node.js, but it’s still quite an immature technology for full stack web development.
(I’m not involved in the Diaspora project)