...it does if all you do is add up the amount of time employees spend on Facebook at work (15 minutes) multiply it by the number of employees who use Facebook at work and divide into the total number of working hours, as this study does, and come up with a productivity toll of 1.5%. Or, you could look at it as a far more complex interaction, and consider that nothing happens in a vacuum. If Facebook weren't aggregating all of my potential daily social activity on one page, for one 15-minute 'burst,' how much time would I spend on other, unproductive activities? You can't simplify the impact of at-work social network usage like this and hope to even begin to capture what this behavior really "costs." And productivity isn't a 'constant' across professions--who's to say that a draconian policy forbidding social network site usage on the job doesn't contribute to more "presenteeism?"
Someday, there will be a proper accounting of the impact of Facebook usage on productivity, but unless the authors of this study have a "control" study showing how productive workplaces are that ban Facebook and other distractions, this one doesn't cut the mustard. Time spent does not necessarily equal time lost.
Besides, more than half of my Facebook connections are business contacts anyway--so what do you do with that information?