Zuckerberg's Harvard classmate Eduardo Saverin was the cofounder of Facebook. He managed the business side of Facebook until 2005, when Zuckerberg boxed him out by creating a new Delaware corporation to acquire Facebook's old Florida LLC, distribute new shares to everybody, and leave Saverin out.
Zuckerberg wrote an email to his lawyer asking, "Is there a way to do this without making it painfully apparent to him that he's being diluted to 10%?" about Saverin.
What followed were lawsuits from Facebook and Saverin, Saverin approaching the Winklevoss twins, and Saverin approaching author Ben Mezrich about the book that would become Accidental Billionaires, which would eventually be made into the film The Social Network by David Fincher and Aaron Sorkin in 2010.
When Saverin and Facebook's lawsuits were settled, Saverin signed an NDA and ceased communication with the press. In a 2012 interview with Veja, a Brazilian magazine, Saverin said, "I have only good things to say about Mark, there are no hard feelings between us." In a 2019 interview with Forbes, Saverin said, "I'm incredibly proud of what Mark has done, to build an institution of its size and value. He'll work hard to get things right."
Sources: Business Insider, Business Insider, Business Insider, Forbes