Jan Koum, Facebook Inc. director and co-founder of its WhatsApp unit, is leaving the messaging service after what people familiar with the matter described as disagreement over putting advertising on its app and frustration over the confines of working in a big company.
Mr. Koum, who was chief executive of WhatsApp, gave no reason for leaving in a Facebook post on Monday about his departure. The move follows the departure last year of fellow WhatsApp co-founder Brian Acton, who has since been publicly critical of the social-media...