I am an old curmudgeon who hated working in an open office (at Google in Mountain View) for a few years.

My personal theory is that the open office trend at Google (and probably Facebook, and others) was initially driven by the fact that the founders jumped directly from school, and never had a "real job" with a normal, old-school office when they were first starting.

Thus they have never personally seen the immense value of a real office, but they HAVE seen the value to the "computer lab" sort of bullpen environment that was common in the 90s (when almost everybody had to show up to the lab to get their work done, as very few people had *nix machines elsewhere where they could do their assignments).