Backbone isn't in any way stale, it's very alive and kicking. However I do not see much exposure on it, compared to Angular or React. I suppose that's because Backbone's core idea remains the same, which Angular changes quite drastically from version 1 to 2, and React with it's virtual DOM being super hip.
The community of Backbone is responsive, but in fact you generally don't need much support because the solution you make are your own and is probably not right or wrong. Backbone's lack of opinion is really a great thing (most cases).
There's a Backbone channel here https://gitter.im/jashkenas/backbone and you can also check out the IRC channel. The Google+ community posts new cool ideas every day. The annotated source code is up to date, as well as the documentation. Marionette and other frameworks are extremely alive (https://gitter.im/marionettejs/backbone.marionette) with lots of discussions going on.
That said, the other frameworks mentioned here all have their benefits and use cases. Just wanted to say that Backbone shouldn't be rejected because people think it's stale - it's not - you should choose the framework for your needs.