On Monday, Mr. Zuckerberg tried to play down any rivalry with Google, saying Facebook's new message service "isn't a Gmail killer" because it won't replace inboxes where people receive bills, receipts, newsletters and other formal emails.

WSJ: Facebook's New Front in Google Rivalry

This would be a killer tool and keep Gmail relevant with the younger demographic as messaging becomes more prominent and emails become less and less relevant.

FORBES: An Open Letter To Google About Gmail