Twitter’s Periscope has surpassed 10 million users just four months after launch, according to a blog from co-founders Kayvon Beykpour and Joe Bernstein, who say they’re seeing over 40 years’ worth of video watched per day. That user base is impressive, Beykpour and Bernstein acknowledge, but add it’s not the metric for assessing their overall success. “Time Watched serves as a proxy for active user growth, without suffering some of the limitations of focusing exclusively on a metric like Daily Active Users (DAU) or Monthly Active Users (MAU),” they point out. “If we were motivated to grow DAU, we’d be incentivized to invest in a host of conventional growth hacks, viral mechanics, and marketing to drive up downloads…this direction doesn’t necessarily lead to a better product.” Adobe recently released data showing that Periscope has generated 3x more traffic than competitor Meerkat since late March.