The tech-blogging world woke up last week to what we noticed a few months ago: Vevo’s sharp and sustained web-traffic fall since the middle of last year, according to comScore numbers. When we followed up with Vevo in a recent report, the company pointed to mobile and tablet growth as one key reason. Now it’s been backed up by Union Square Ventures VC Fred Wilson, who identifies “a significant shift going on this year, much more significant than we saw last year, from web to mobile”. Or more plainly: “Web is flattish. But mobile is growing like a weed.” Which poses problems for audience measurement, which tends to be one or the other.