In an unexpected move, Instagram co-founders Kevin Systrom and Mike Krieger resigned from the photo-sharing app – which is owned by Facebook – on Monday.

Facebook acquired Instagram in 2012 for about $1-billion when the team was only made up of 13 employees and 30-million registered users.

The app has since grown from 30 million users in 2012 to over 1-billion in 2018, proving to be one of Facebook’s most lucrative acquisitions.

Systrom, who was Instagram’s chief executive officer, and Krieger, who held the position of chief technical officer, did not give reasons for stepping down but there is speculation that the duo left because of growing tensions between them and Facebook Inc chief executive Mark Zuckerberg.

According to Bloomberg, the pair had succeeded in running Instagram as a mostly independent entity while using Facebook’s infrastructure to grow but Zuckerberg is reported to have recently upped his involvement in the operations of the photo-sharing app, which Systrom and Krieger were against.

In a blog post, Systrom expressed his gratitude to Facebook and Instagram teams, saying: “We’ve grown from 13 people to over a thousand with offices around the world, all while building products used and loved by a community of over 1-billion.”

Systrom added that he and Krieger would be taking time off to “explore our curiosity and creativity again” and reflecting on how “building new things” requires stepping back and understanding “what inspires” them. 

In a statement, Zuckerberg described Systrom and Krieger as “extraordinary product leaders” saying he had “learned a lot working with them for the past six years” and enjoyed the experience.

“I wish them all the best and I’m looking forward to seeing what they build next,” Zuckerberg said.

Facebook has parted ways with several executives of companies which it has acquired over the past year or so.

In November 2017, WhatsApp co-founder Brian Acton left Facebook over alleged disagreements over the messaging app’s business model.

Acton’s partner Jan Koum followed suit in April 2018 over clashes related to user data privacy and encryption with Facebook management.

WhatsApp was acquired by Facebook in 2014 for $22-billion and has become one of the world’s leading messenger apps.