Facebook claims 200M+ active users for its Messenger service, and recently took the drastic step (in the name of creating a better experience) of requiring mobile users to download a standalone Messenger app to use the service.
As with many other products, Facebook has been cautious with its efforts to monetize Messenger. Marcus' hiring could signal a strategy change.
Marcus won't be running WhatsApp. Facebook has promised WhatsApp will be run independently; co-founder Jan Koum is currently its CEO.
Marcus has been trying to change PayPal's once-cautious culture since becoming head of the online payments giant in 2012, as well as expand its mobile footprint and improve its customer service (often a target of criticism). He admitted earlier this year a culture clash has formed between PayPal and eBay as he pushed ahead with his changes.
The PayPal team will report to eBay CEO John Donahoe until Marcus' successor is named.