The other day Evan Williams a former CEO of Twitter and now the head of Medium, posted a rant on Medium, the publishing platform he helped create. It caught our eye because it touched on a hot topic: Monthly Active Users. That’s the number of people who interact with your service or your platform at least once a month.

If you’re a social media company in 2015, especially one that has gone or is going public, there’s a good chance you’re talking a lot about monthly active users. This number is used to measure the success of companies like Twitter or Buzzfeed. But Williams doesn’t think that’s the only metric for success. 

What kicked off his rant in the first place was a question about whether Instagram is bigger than Twitter because it has more users. But what exactly does bigger mean? Williams says it’s frustrating that so many people measure the success of consumer internet services — news, social media etc — solely by number of users. Twitter’s CEO Dick Costolo is facing investor criticism right now because the social network isn’t seen to be gaining new monthly active users fast enough.

Williams says that single number — the number of people who visit or use a site at least once a month — isn’t a fair metric. For one, it doesn’t tell you how many people spent less than a minute on the site, and how many stayed longer. “What value are you measuring, either to the people or to the company?” he says.

Williams thinks time is “one of the other dimensions worth paying attention to,” but it’s an imperfect way to measure a site’s impact on people.

“The ultimate metric is probably not traceable,” he said. But for now, he’s interested in defining what bigger means.