"Time online per week seems plausible from personal experience, but I didn't expect the average to be so high."

I personally spend an average of 50 hours a week online.

That's because, by profession, I am a web-developer.

The percentage of LessWrong members in IT is clearly higher than that of the average population.

I postulate that the higher number of other IT geeks (who, like me, are also likely spending high numbers of hours online per week) is pushing up the average to a level that seems, to you, to be surprisingly high.