WOW this is awesome. I know I implemented this feature but I haven't actually played around much with Facebook auth myself. I've been testing with Twitter, Tumblr, Github, Wordpress, Medium, Reddit, etc. but for some reason dont' have a single JSON example for facebook. So it would be great if you could share the JSON code.
As for why it's not documented: First of all, like you noticed, the iOS $oauth action is actually pretty functional as is. I could have documented and officially supported it if I wanted to (I even have several JSON examples). But when I was finishing up the iOS version and was about to open source it, I thought about this. Back then, a lot of things were unclear--including how long it would take to build even a basic working Android version (There was 0 code for android just a month ago and I started from scratch)--and since my goal was to get to the point of supporting both iOS and Android asap before moving forward feature-wise, I decided not to introduce another huge complexity that is Oauth, until we reach the Android release.
Which means, now that both iOS and Android are out, I think now is a good time to revisit. I just couldn't find time to think about this lately since I have been mostly focused on fixing lower level and cross platform related stuff for Jasonette.
So I would totally appreciate it if you can contribute in anyway with oauth. Especially the Android OAuth would be awesome. We can also document the oauth action and release it officially as we go. As you can imagine, once this is implemented, this would be one of the most important features of Jasonette.
Maybe we can continue this conversation as a github issue? Or we can also chat on slack if you're there. Whatever works best, thanks!