That was not the point I was trying to make. I block FB at work and saved gigabytes of traffic a day.
A single Farmville session can be 20MB just to load the flash. And don't forget the advertisements and data mining add up.
Even my grandma who checks her stocks and email every third day or so is ~20 GB a month.
True, you can keep it lower with constant vigilance but I don't see a lot of people doing that.