Twitter, which is searching for a CEO after Dick Costolo said Thursday he is stepping down, generates nearly all its revenue from advertising -- 89% last quarter to be exact. This line item grew sharply in the year since the company started trading publicly in November 2013, nearly doubling to $432 million during the fourth quarter of 2014. But the first three months of this year told a different story: ad sales declined.

The company's strategy for returning growth to ad sales, and making sure its number of monthly active users continues to rise, will be the central challenge for its new CEO.

Chart created by Chen Wu.