How can they tell?
Hours per week spent online may have been a meaningful measure when almost all home users were on dial-up connections - but with broadband, surely I'm 'online' whenever I'm sitting at my PC (or, if you prefer to measure it from my DSL router, 24x7).
These guys need to decide what they want to measure. Is it time spent using a web browser, playing interactive games, using IM, downloading media files, reading email? All of these are much more difficult to find out than 'time online', but at least they might mean something!