Just wondering how about those people whose job is to actually monitor social media sites, or are the social media personnel for some companies? Their time obviously should not fall under the distracted category.
And also, even for writers, there would be occasions where you'd need a peek at social media sites to gather some inspiration. Or if you are writing about a topic, and social media becomes your source for relevant information, then that as well should not be construed as wasted time.
Will there be a way to have an objective measurement of every person's productivity? Collect all data based on outcomes desired for each task? At the moment, there's just too many variables at play to even consider measuring productivity.