When Twitter announced earlier this month that CEO Dick Costolo would step down, the company said his resignation would be effective July 1. That day has come.

Costolo took the reins at Twitter after a long period of instability in its upper management. Jack Dorsey stepped down as CEO in 2008, with cofounder Ev Williams replacing him; in 2010, Ev Williams stepped down, and Costolo replaced him. “Seems like a revolving door [there],” an interviewer said to Dorsey at a Columbia Business School event in 2011.

Costolo lasted the longest in the job thus far. In his five years as CEO, Twitter has gone from a private tech startup uncertain how to make revenue to a publicly traded company with a market cap of nearly $24 billion. The revenue part is still somewhat uncertain—and so is Costolo’s legacy.

As Fortune‘s Erin Griffith wrote, Costolo started off swimmingly at Twitter and was beloved by Wall Street. He “shook things up,” replacing the entire board and hiring Adam Bain to create a strong ad business. Employees loved him—they congratulated and thanked him on Twitter using the hashtag #thankyoudickc, and there were still new tweets showing up on Tuesday.

But after a jubilant IPO, Twitter’s shares took their first major hit on January 9 of last year. As Griffith wrote: “Twitter has been playing defense ever since.” The stock is up just a bit, nearly 2% in the past five days, but has major ground to gain back. Its all-time high was $69 on January 3, 2014.

Costolo granted an exit interview to The Guardian on Tuesday. (And he penned a separate op-ed for the site.) He discussed the pressure that comes with going public. He also told the paper that leading Twitter had become political, “citing for example Iran, where authorities use the site to communicate while blocking citizens’ access.” He added that although Twitter cannot put a stop to all political problems, he did see it grow into a powerful tool, and that “governments now found it harder to cover up the true version of events.”

The company has yet to name a permanent successor; Jack Dorsey is serving as interim CEO. Costolo will continue to serve on the company’s board.

Correction: An earlier version of this story stated Wednesday is Costolo’s last day as Twitter CEO. Per a Twitter spokesperson, Costolo’s last day as CEO was Tuesday, while his resignation is effective Wednesday.