Facebook confirmed to CNBC that Koum is still an employee, despite an April Facebook post in which he said it was time to "move on."
But The Wall Street Journal reported Wednesday Koum hasn't shown up since July and is instead taking advantage of a practice commonly known in Silicon Valley as "rest and vest" in which executives remain on the payroll and wait for stock options to free up. Koum said his goodbyes back in May but has yet to fully leave, the newspaper reported.
A Facebook spokesperson said Koum is employed and working on "various projects and helping transition WhatsApp leadership."
Facebook bought WhatsApp for $19 billion in 2014, its largest-ever acquisition. Koum's corresponding employment offer included restricted stock units with a four-year quarterly vesting period, meaning those options wouldn't be free until the end of 2018.
Each quarterly payout from the stock options has earned Koum in the neighborhood of $400 million, based on variations in Facebook's stock price. His last stock award of more than 2 million shares is set to vest in November, WSJ reported, and would be worth roughly $350 million as of Tuesday's stock price.
Koum's planned departure would have meant both WhatsApp founders had left Facebook in the years since the sale. The messaging service's other founder, Brian Acton, left in 2017.
Read the full story from The Wall Street Journal.