As Facebook eyes its messaging apps Messenger and WhatsApp to drive future growth, CEO Mark Zuckerberg is focused on a company that isn't even in the social media space: Apple.

"Our biggest competitor by far is iMessage," CEO Mark Zuckerberg said on an earnings call on Tuesday with investors, referring to the messaging service built into the iPhone and other Apple products.

While Facebook's messaging services are stronger in places like Europe, where iOS and Android sales are closer, it still is losing out in "important" territories like the U.S. where iPhone sales dominate, according to Zuckerberg.

"In important countries like the U.S. where the iPhone is strong, Apple bundles iMessage as a default texting app and it's still ahead," he said.

Apple CEO Tim Cook has been harshly critical of tech companies that use people's personal information to profit. Without naming Facebook directly, Cook criticized the company's business model in a recent speech about privacy in Brussels.