Later in the article: "Among WhatsApp’s competitors is Signal, an encrypted messaging app run by a nonprofit called the Signal Foundation and dedicated to secure communication, with strict privacy controls and without advertising. Mr. Acton donated $50 million to fund the foundation and serves as its executive chairman."
"When Mr. Acton departed Facebook, he forfeited about $900 million in potential stock awards, according to people familiar with the matter. Mr. Koum is expected to officially depart in mid-August, in which case he would leave behind more than two million unvested shares worth about $400 million at Facebook’s current stock price. Both men would have received all their remaining shares had they stayed until this November, when their contracts end.Behind the Messy, Expensive Split Between Facebook and WhatsApp’s Founders -- WSJ
The amount the two executives are leaving in unvested shares hasn’t been reported, nor have the full extent of the details around their disagreements with Facebook over the years."