Forgetting for a while the non-linear costs of distraction in cognitively intensive jobs, I wonder if Facebooking at work is really costing us anything at all? I'm having a hard time imagining that office workers of 20+ years ago were that more job-focused than they're now.
But then again, the problem may be Internet in general - I can't imagine people being able to do so many non-work errands while in the office if it weren't for e-mail in particular.
I did data entry in an office with no internet connectivity (well it did, but they didn't allow browser usage, we connected to mainframes to do our work) before about 12 or 13 years ago.
When people wanted to zone out they read books, usually. Audiobooks were also common. I brought the entire series of Discworld novels in on pdf, and managed to read the entire series while I worked there. I also wrote a bunch of notes down on paper.
Hell, when I was working in warehouses away from computers entirely I'd still zone out during it and mentally work out some ideas for personal projects once I was done with work. Sometimes I actually miss that, since I seemed to have a lot more ideas then.
Point being, you don't need computers or internet for people to find ways not to be 100% productivity machines.
But then again, the problem may be Internet in general - I can't imagine people being able to do so many non-work errands while in the office if it weren't for e-mail in particular.