Another kind of efficiency is the pure real-estate economics angle. Open office plans typically allocate fewer sq ft per employee, are cheaper to maintain, and are more easily reconfigurable as divisions/headcount changes.

This is probably less of a factor for Facebook, who built a bespoke office building and have a ton of cash, but it's definitely driving things elsewhere in the real-estate market. If you read what's being written in the architecture / office-consulting business, there is much touting of the cost/flexibility advantages of open-plan offices compared to "legacy" floorplans. Even boring old companies like ExxonMobil are moving towards open-plan offices in new construction mostly for this reason. And office buildings built speculatively by investors in the past 20 years almost never have actual structural offices like older buildings do, because that's seen as producing a rental inventory that's less flexible. Companies leasing space just get a big open floor space, and then have a choice between setting it up in open-plan or cube-farm style.

Sure, now you know when people are at their desks, which is irrelevant. You want to know when people work and that doesn't follow from "is at the desk".