I work in a computer lab on my university campus. I sit in the lab for 3-5 hours a few days a week. With this kind of time, I have noticed many people come in and read/post on facebook for at least an hour every day (I have seen people using facebook for my entire shifts).
When I notice a line starting for computers, I go around and ask the people I noticed spending too much time on facebook to let those in line have a chance. I always do this politely but even though they have spent an hour or two on facebook, I am always met with a look of disgust. Some of those people have even started yelling at me and others I've had to call campus police to escort out. It really seems as if many people have a mental addiction to social networks like facebook. They fear that they will miss something important if they aren't watching the news feed 24/7.
Those are the type of people that I see when I'm working the lab that lead me to believe these statistics just a bit. 16 billion seems too big but I can believe at least 3-5 billion minutes are spent on facebook a day just from the people I see in the labs.
Now something I do agree with you with respect to the pictures viewed is that facebook may also be counting the time people are logged into the facebook chat or just have a facebook window open. I know I leave firefox open all the time with facebook somewhere in my assortment of tabs.