Instagram's co-founders are leaving Facebook

Instagram co-founders Kevin Systrom and Mike Krieger have resigned as chief executive officer and chief technical officer.

The two announced that they would be stepping down from their posts at the photo-sharing platform, but they didn't explain why.

Instagram, which Facebook bought in 2012 for 1 billion dollars, has more than 1 billion active monthly users.

It's also Facebook's fastest-growing slice of revenue.