If all goes as planned, we want to have an initial beta of sorts available around GDC. What platforms or to what extent this beta means I’m not 100% sure at the moment, but will keep you guys updated.
At launch we will not supply any extensions ourselves, but instead some examples and getting started manuals/tutorials so the community can start creating/experimenting. But in the long run we want to “break out” some of our internal “extensions” (as you mentioned for example Facebook) to be real extensions. This means somewhere down the line you will be able to just depend on the official Facebook extension as a library, or a third party one, and not be forced to have it in the binary as you currently are. 
There will be a couple of new scripting modules with the extensions system, one example is the resource.* which can be used to update resources at runtime. Currently we are experimenting with texture resources, which our video player extension uses to update a texture with the video output in runtime.
We want to add more resources that can be changed at runtime than just textures, like meshes and sound, but I’m not sure how many will be available in the first release. (Not sure about drawing/rendering and GLES state stuff.)