Zuckerberg himself touted the value of Instagram and WhatsApp to the group's bottom line and growth prospects, saying that at least 2.5 billion people use at least one of its four messaging platforms each month and that Instagram hit a "milestone with now more than 1 billion actives". Mobile ad revenue, which accounts for more than 90% of Facebook's overall sales, rose 50% in the second quarter (compared to last year) to $11.9 billion, "driven primarily by ads in feed on Instagram and Facebook", according to CFO David Wehner. The exit of Systrom and Krieger is the second major executive departure for Facebook in the past six months and follows the resignation of WhatsApp co-founder and CEO Jan Koum, who announced his decision to quit in April amid reports that he and co-founder Brian Acton clashed with Zuckerberg over Facebook's desire to use the personal data of WhatsApp's 1.5 million daily active users to form target ads that would increase group revenue.