This afternoon Twitter's Chief Operating Officer Adam Bain announced in a series of tweets that he is leaving the company after six years "and a once-in-a-lifetime run." Listed second under CEO Jack Dorsey on Twitter's management page, Bain was in charge of building the company's advertising business. His departure comes after weeks of turmoil for the company, including rumors of a sale and hundreds of layoffs. A Bloomberg report in October signaled an internal struggle with CFO Anthony Noto taking more control, and he will replace Bain in the COO position.