Ev Williams (left) with Fortune senior editor-at-large Adam Lashinsky

FORTUNE — Where online publishing is concerned, Ev Williams is on a roll.

The 41-year-old serial entrepreneur developed, then sold the early blogging platform Blogger to Google in 2003. More notably, he co-founded Twitter TWTR with Jack Dorsey and Biz Stone, an origin story fleshed out in New York Times columnist Nick Bilton’s bestseller, Hatching Twitter. With his most recent venture, Medium, Williams, Stone, and former Twitter exec Jason Goldman are after digital publishing again, this time shirking the 140-character constraints for a long-form-minded platform of idea-sharing and storytelling.

“We call them ‘ideas’ and ‘stories,’ but it can be personal stories or it can be investigative journalism. It can be two paragraphs or 50,000 words,” explained Williams to senior editor-at-large Adam Lashinsky Thursday evening at Fortune’s Brainstorm Tech dinner in Menlo Park, Calif. “On the creation side, it’s the easiest way to put a thought out in the world or a story.”

VIDEO: How Medium stands apart from its competitors

Launched in 2012, Williams says Medium is growing quickly: The platform’s userbase ramped up from 9 million unique monthly visitors in December 2013 to 13 million visitors currently reading items like Susie Cagle’s “San Francisco’s Class War, By the Numbers” or “The problem with false feminism,” by Dani Colman. Williams also revealed that an iOS app is due to launch next week that will focus on consuming content on Medium. (Users won’t be able to create content through the app yet.) Williams declined to give further details, other than to say he believes the platform “is going to be really big,” words he also once used to describe Twitter during its early days.

Williams also remains a Twitter board member and recalled reading Hatching Twitter, the critical origin story of the now-popular social network. In fact, Twitter read the book on a flight en route to the company’s IPO in New York. He recalled the experience as “very intense.” “I mean, the book starts with me puking in a trash can,” he elaborated. “But it ended with me reading books with my wife to our kids, which was lovely.”