The Facebook PR will get everyone noticing MU-MIMO. The top wireless engineers yawned. They have known for years that you could get extraordinary results from adding antennas and multiple streams. Bristol used 128 small antennas in about eight feet by four feet for a 12X efficiency gain. Facebook used 96 antennas (pictured) for what they considered a 10X gain. These are very early units. Better software and dedicated processors will take speeds higher; Facebook promises ~40% improvement. (Many details and excellent video below.)
20 MHz of spectrum could deliver a gigabit. Early LTE networks, like AT&T, get ~100 megabits. More recent LTE-A networks in Turkey, the Philippines, and many other countries use 60 MHZ and will get speeds well into the gigabits.
An important caveat: MIMO works because the signals from each antenna can be separated by how they bounce off walls, etc. That means that the MIMO gain is much less in some locations. Line of sight is best for today's systems but not ideal for MIMO. These are ideal figures; at the edge of a cell you get much less.
Arogyaswamo Paulraj invented MIMO in 1993. He predicted twenty years ago that you would one day be able to increase efficiency 100X.