The messaging service announced the latest tally Wednesday, about six years after Facebook Inc. acquired the app for $19 billion.
WhatsApp is the largest messaging platform in the world, though it makes virtually no money and recently set aside plans to introduce ads.
WhatsApp also defended its use of encryption just a few months after clashing with politicians in Washington over the technology.
Facebook argues that encryption, which prevents anyone, including Facebook, from reading users' messages, is good for user privacy, though it also makes it harder for law enforcement to track and monitor some criminal behavior.
"Strong encryption is a necessity in modern life," according to a WhatsApp blog post. "We will not compromise on security because that would make people less safe."
Facebook is working to encrypt all of its messaging apps, including Messenger and Instagram.
