"Private conversations that once were only possible face-to-face can now take place across great distances through instant chats and video calling," a WhatsApp blog post said.

"There are so many significant and special moments that take place over WhatsApp and we are humbled and honored to reach this milestone."

The statement said WhatsApp remained committed to its "strong encryption" that enables users to connect privately even amid calls by law enforcement in the United States and elsewhere to provide more access.

"Strong encryption is a necessity in modern life. We will not compromise on security because that would make people less safe," WhatsApp said.

"For even more protection, we work with top security experts, employ industry leading technology to stop misuse as well as provide controls and ways to report issues — without sacrificing privacy."

Last week, child protection organisations called on Facebook to halt plans for strong encryption of all its platforms, saying that would allow predators to operate freely.

WhatsApp employs "end to end encryption" which can in many cases prevent law enforcement from accessing user data even with a court order.

The social network is working to extend end-to-end encryption across its messaging applications, including Facebook Messenger and Instagram.

Back door dilemma

Child protection groups have expressed fears that stronger encryption of online exchanges would facilitate the sharing of child pornography.

Backers of strong encryption argue that any special access or "backdoors" allowed for law enforcement would weaken security and could be exploited by criminals, hackers and authoritarian governments.

Officials from the US, Britain and Australia late last year called on Facebook to allow authorities to circumvent encryption to better fight extremism, child pornography and other crimes.

The heads of Facebook's WhatsApp and Messenger, Will Cathcart and Stan Chudnovsky, responded in a letter to officials from the three countries that allowing this kind of "backdoor" access "would be a gift to criminals, hackers and repressive regimes" while leaving users vulnerable.

Facebook's stance on encryption has been backed by more than 100 activist organisations, security experts and industry groups who warned against efforts to force tech companies to weaken encryption.

Despite its strong encryption, WhatsApp has seen flaws exploited in cyberspace.

Human rights activists have said that spyware hidden in WhatsApp messages, possible developed by Israel-based NSO Group, was used to track dissidents and others.

Amazon chief Jeff Bezos's phone is also believed to have been infected by spyware hidden in a WhatsApp message from Saudi Crown Prince Mohammad bin Salman.

WhatsApp in October sued NSO Group, accusing it of using the messaging service to conduct cyberespionage on journalists, human rights activists and others.

Breakup?

WhatsApp is one member of the Facebook app "family" that includes its core social network, Instagram and Messenger.

Facebook said recently some 2.89 billion people globally are daily users of at least one of these services.

But the growth has also attracted attention of regulators and activists concerned over the dominance of major tech platforms. Presidential hopeful Elizabeth Warren has been among those calling for the breakup of the big technology firms.

Facebook has argued against the idea of a breakup, saying the company is better able to keep its services safe and secure with a unified infrastructure.