Has there been any research into OpenBTS, Osmocom, OpenLTE ?
This is really an opportunity for Facebook to show that they are serious with their efforts to bring Internet to developing regions.
Especially with the delayed adoption of Cellular tech generations on the cellphone side, I think an open source Stack is the only way to go.
If the networks are built using decommissioned tech from somewhere else, it will be brittle and won't be cost effective in the long run, especially considering energy consumption.
The only missing part now is a baseband implementation with GSM/UMTS/(LTE) support.