Or just play a full screen slide show of various spreadsheets and documents. Will they really study the screenshots that closely to see the repetition?
In my experience employers only tend to look back at things like this once they have a “reason” and are trying to document/justify performance issues. Although I agree it doesn’t seem too far off to have a system that proactively analyzes the screenshots and alerts a manager when Facebook or gmail seems to be open a disproportionate amount of time.
(Not to say I endorse this means of tracking employees' time, I just don't think that's a big impediment to this method.)