What is the difference between this and Firebase?
Firebase allows you can listen to any part of the subtree and query particular children of a node. Data is synchronized in realtime, optimistically, and the events that fire allow you to easily tie your UI or Backbone model to the datastore. It also has critical features like transactions. This is much more flexible than what Spire.IO currently provides.
Firebase doesn't have an identity service, does it? Firebase doesn't seem to address the issue of security or privacy at all, which seems to me to be fairly important to a web application.