I'm trying to model an infinity mirror but can't seem to get the reflections working correctly. I have a simple node setup of a glossy and transparent into a mixed node and the geometry node connected using the backfacing node to the FAC of the mix shader. I only have one "layer" of reflection ... how can I get the mirroring effect of infinity.
I think your node setup might be reversed, you probably want the glossy shader in the upper slot of the mix shader.
Anyway it may not work that way, if I understand correctly you may need a reflective 'lid' on your box so there's something else for the ground to reflect. Also since 'inside' your box it's dark so there is no light to reflect. Even though it's backfacing is transparent seen from inside it is all mirror so no light is getting in. If you add a light to the inside of your box it might work.
One side note if you are aiming for a 'infinity mirror' effect you may want to bump up the number of reflective bounces for cycles under the Properties Window > Render Tab> Light Paths > Bounces Glossy otherwise after that number of bounces cycles will return black instead of the actual reflection.