Basically, a web app that is usable (or at least readable) before all your JavaScript initialization is done. With Facebook, as soon as the HTML and CSS hits your browser it can render a more or less complete page. With Ember.js, that doesn't happen until the application is initialized and all your templates are rendered.
To be fair, that totally depends on how the application is written. You can have server-side-rendered page with single-page application, with or without Ember.js.
With Ember.js the routing framework is client-side and Ember.js stores all dynamic values in the URL fragment. The server doesn't see enough information to render a page, even if you were willing in theory to duplicate your rendering logic server-side.
You can use HTML5 pushState in Ember.js [1] and Backbone so application state is captured in actual URL path instead of URL location hash. The server can see enough information to render a page.
Basically, a web app that is usable (or at least readable) before all your JavaScript initialization is done. With Facebook, as soon as the HTML and CSS hits your browser it can render a more or less complete page. With Ember.js, that doesn't happen until the application is initialized and all your templates are rendered.