Facebook shares dropped 2% in premarket trading on Tuesday after the co-founders of Instagram, the company’s fast-growing photo-sharing app, decided to leave Facebook under mysterious circumstances amid alleged clashes with CEO Mark Zuckerberg, deepening an executive exodus that has intensified over the past six months as Facebook struggled with accusations of Russian interference and the rolling fallout of the Cambridge Analytica scandal.

According to the Financial Times, Instagram founders Kevin Systrom and Mike Krieger are leaving the photo-sharing app that they sold to Facebook for $1 billion back in 2012. With the departure of Systrom and Krieger, the founders of Facebook’s biggest acquisitions have now left the company.

Their departure comes less than six months after WhatsApp’s founders Jan Koum and Brian Acton quit the US social media group, after clashing with Facebook chief executive Mark Zuckerberg over privacy and data protection in the wake of the Cambridge Analytica scandal.

Including the departure of Oculus VR co-founder Palmer Luckey 18 months ago, the founders of Facebook’s three biggest acquisitions have now all left the company.

Bloomberg provides some context to the departure, reporting that Systrom and Krieger had been able to keep the brand and product independent while relying on Facebook’s infrastructure and resources to grow. But lately, “they grew frustrated with an uptick in day-to-day involvement by Zuckerberg, who has become more reliant on Instagram in planning for Facebook’s future.”

The departure of Systrom and Krieger follows several high-profile executive departures at Facebook, including that of Chief Security Officer Alex Stamos, who famously suggested that the company was pressured into finding “evidence” pointing to Russia’s purported interference in the 2016 election, and Product Chief Chris Cox, who was put in charge of the “family of apps” that includes Instagram, WhatsApp, Messenger and Facebook.