[wmf]

If you just care about capacity and cost you can build a system with a $20 ARM SoC for every two DIMMs. The ultimate is BlueGene-style packaging with an SoC on each DIMM, but Facebook's probably not ready for that.

[duskwuff]

Other direction you could go would be to put a bunch of the RAM on a peripheral bus (e.g, PCIe).