Instagram CEO Kevin Systrom said yesterday that the photo-sharing service has overtaken Twitter in active users, but Ev Williams doesn't seem too concerned. The Twitter co-founder, who also started Blogger before Google bought it out and now spends his time on projects like Medium, tells Fortune that he "doesn't give a shit" about the services' respective userbases in terms of pure numbers.

"Think about the impact Twitter has on the world versus Instagram."

"It's a question of breadth versus depth. Why is users the only thing we talk about?" asks Williams. "If you think about the impact Twitter has on the world versus Instagram, it's pretty significant. It's at least apples to oranges. Twitter is what we wanted it to be. It's this real-time information network where everything in the world happens on Twitter — important stuff breaks on Twitter and world leaders have conversations on Twitter. If that's happening, I frankly don't give a shit if Instagram has more people looking at pretty pictures." Williams also notes that "Twitter makes a hell of a lot more money than Instagram, if that's what Wall Street cares about."

Twitter is indeed doing a pretty good job of making money out of its existing users, but its financial performance is more solid than spectacular. And user growth is slowing, which may spook investors; last quarter, the company reported a 23 percent increase in monthly active users, down from around 50 percent year-on-year growth in 2013.