Evan Williams, Twitter's largest individual shareholder with 10 percent of the company post-IPO, was CEO until he was ousted by the board in 2010. He was replaced by Costolo, who was brought in as COO the prior year.
Williams co-founded Obvious Corp., an incubator for Web media companies and an investment firm of sorts, along with Twitter's former chief creative director, Biz Stone, who left Twitter at the same time as Williams.
(Read more: Twitter's mysterious outside shareholder)
Obvious' main project is a publishing platform called Medium, which launched last year. Medium offers an alternative—almost an antidote to Twitter's 140 character limitations. It's designed for easily sharing and reading longer-form content—magazine-style articles. It helps writers find an audience and helps readers find relevant content, enabling them to comment and share with friends.