Six years after selling their startup to Facebook, Instagram's founders have announced their departures from the company, setting off speculation in the tech world as to what might have prompted the exits and what could now lie in store for the photo-sharing platform.
Systrom is chief executive and Krieger chief technical officer of the photo-sharing app, which they founded in 2010 and which was acquired by Facebook for $1 billion (£760m) in 2012.
It became the fourth Facebook platform to eclipse the billion-user mark, including the namesake social network with more than two billion users, and the messaging applications WhatsApp and Messenger.
Instagram, which was purchased by Facebook in 2012 for $US1 billion in cash and stock, now has more than one billion users. Facebook users also have been flocking to Instagram as an escape, exhausted of the political bickering and privacy scandals that plague the parent company.
Systrom and Krieger submitted their resignations to Facebook chief executive Mark Zuckerberg and plan to leave in the coming weeks, according to a person familiar with the men's decision-making.
Systrom wrote in a blog post on Monday that he and Krieger planned to take time off and explore "our curiosity and creativity again".
Facebook representatives said they had no comment beyond a statement from Mr Zuckerberg on Monday night. At the time Instagram was ad-free, with a loyal following of 31 million users who were all on mobile devices - still a somewhat elusive bunch for the web-born Facebook back then.
The co-founders of Instagram are resigning their positions with the social media company without explanation.
Facebook stock is now down about 6 percent this year, after six years of stellar gains.
Will Instagram disappear? Get cluttered with ads?
As the number of users increased, so has the value of the company.
Fellow WhatsApp creator Brian Acton departed in November a year ago with a more pointed message, which he demonstrated by donating $US50 million to a rival messaging app.
If the Instagram executives left because of tension with Facebook, they made no mention of it in their farewell messages. As Facebook struggles to add more users to the social media platform.
Instagram's simple design - just a collection of photos and videos of sunsets, faraway vacations, intimate breakfasts and baby close-ups - has allowed it to remain a favorite long after it became part of Facebook. WhatsApp's other founder, Jan Koum, left in 2018 also citing issues with Zuckerberg's push for ads and collection of user data as his reason for leaving.
Brian Acton, a cofounder of WhatsApp, has broken his silence about leaving Facebook three years after his company was acquired for $US19 billion.