Facebook Inc. (FB) is undergoing a transition from building out its News Feed service to investing in private messaging over WhatsApp and Messenger, and according to Facebook CEO Mark Zuckerberg, Apple Inc. is the company's biggest competitive threat — by far. Remarks from the Facebook leader during Tuesday's earnings call pit Zuckerberg and Apple CEO Tim Cook, who have a history of public dispute, against each other once again. 

Tech CEOs Criticize Competing Business Models

While Facebook is leading in most countries, particularly in the developing world's Android market where consumers choose their own default messaging app, Zuckerberg reported that the popularity of the iPhone in the developed world has limited WhatsApp and Messenger's market share. 

"Our biggest competitor by far is iMessage. In important countries like the U.S. where the iPhone is strong, Apple bundles iMessage as the default texting app, and it’s still ahead," said Zuckerberg. 

The CEO's use of the word "bundled" traces back to antitrust lawsuits against Microsoft Corp. (MSFT) for bundling computers with Internet Explorer and could be viewed as an attempt to draw regulatory attention to iMessage at a time when Cook has repeatedly fired at Facebook and its privacy practices. 

In a recent speech on privacy in Brussels, Cook criticized the business models of tech companies that rely on users' personal information for profit. 

"Every day, billions of dollars change hands and countless decisions are made on the basis of our likes and dislikes, our friends and families, our relationships and conversations, our wishes and fears, our hopes and dreams. These scraps of data, each one harmless enough on its own, are carefully assembled, synthesized, traded and sold," said the Apple CEO. 

Zuckerberg jabbed back on the Q3 earnings call, stating that, "it's worth noting that one of the main reasons people prefer our services, especially WhatsApp, is because of its stronger record on privacy." He cited various privacy features of WhatsApp, including end-to-end encryption. 

Facebook Doubles Down on Messaging and Stories

Despite Apple's dominance in some markets, the Facebook CEO flaunted the company's success with its messaging services and Stories features, which he says are fueling a "vast majority of growth" in sharing content. Facebook users send roughly 100 billion messages per day, with WhatsApp serving as the medium with more photos, videos, and links than any other social network.

Facebook shares are up nearly 3% on Wednesday morning on the back of Q3 earnings in which EPS beat estimates, yet revenue and active user projections fell short of forecasts.