Bandwidth isn't necessarily the best measurement of "what the internet is used for", for example porn may stream in lower quality on average, maybe more people hoard porn (download once, watch many times) as it's probably easier to remember the name of your favourite film than which random site to find your favourite porno on, plus the porn category includes pictures as well as video.
But I guess what the best measurement would be is subjective anyway, plus it's not so easy to measure anything like "amount of time spent doing X" or "number of people doing Y". Alternatively it's a valid opinion to have that bandwidth is the best metric, it's just not my opinion.