The move toward an all-encompassing messaging platform has raised concerns from experts about both privacy and security for users and their data as well as antitrust issues if the company combines its platforms.
As an example of how this would work, Zuckerberg wrote, "lots of people selling items on Marketplace list their phone number so people can message them about buying it. That's not ideal, because you're giving strangers your phone number. With interoperability, you'd be able to use WhatsApp to receive messages sent to your Facebook account without sharing your phone number."
Currently WhatsApp users must connect through a phone number.
Interoperability, or the ability to message fluidly across all of the company's platforms has been mentioned by Zuckerberg before, but this is the most definitive statement the company has made about how it will work. It will be opt-in, Zuckerberg wrote, but that still has privacy and anti-trust experts concerned.
Regulatory experts told ABC News that Facebook's apparent goal of having one behemoth messaging platform for social networking, ads and commerce integrated into daily interactions like China's WeChat — in which a user can order and pay for dinner — could be considered a monopoly. It would also give the company access to a tremendous amount of user behavior.
"Antitrust authorities are increasingly concerned that as people begin to leave Facebook, Facebook is buying up their competitors — Instagram and WhatsApp in particular. Even if Facebook's users don't like its privacy practices, if they leave for Instagram or WhatsApp, they are just coming back in the building through a different door," Blake Reid, a professor at the University of Colorado Law School, told ABC News.
"There is some talk about reversing the approval of Facebook's acquisitions," Reid said. "But if Facebook's messaging applications are integrated with Instagram and WhatsApp, separating the companies becomes much more difficult."
Ashkan Soltani, former chief technologist for the Federal Trade Commission (FTC), agrees.
"It's a judo move," Soltani told ABC News. "Taking your enemies' force toward you, redirecting for your advantage. By using privacy and security as a cover, it counter-veils the antitrust issues raised. Once users start using it and see how easy it is to text your mom from Instagram, it will dispel any public backlash [if authorities try to impose bans]."
Facebook is currently facing 10 open investigations in Europe for violating the General Data Protection Regulation (GDPR) rules, which went into place last May, including one restricting WhatsApp from sharing data with Facebook, a European Union official confirmed to ABC News.
Germany's antitrust regulator, the Bundeskartellamt, ruled in February that Facebook cannot share messaging across platforms or data it collects with third-party applications without a user's explicit consent.
"The extent to which Facebook collects, merges and uses data in user accounts constitutes an abuse of a dominant position," the Bundeskartellamt wrote in a statement announcing its decision. Facebook is appealing that decision.
"Using information across services helps to make them better and protect people’s safety," Facebook said in a blog post. It also denied it was a monopoly by writing, "Popularity is not dominance."
"But we already know in Europe, this is what Germany asked Facebook not to do and the FTC and several state attorneys general have confirmed they are looking into Facebook," Soltani said. "Outside of fines, which the FTC can kind of do, states don’t have the ability to issue a big enough fine to deter the company. But they can move for injunctions to block certain activities."
Facebook said that monopoly and privacy "are two separate issues and shouldn’t be conflated."
In his post and his last few calls with journalists, Zuckerberg has repeatedly stressed the security in end-to-end encryption, which is available on WhatsApp, and said that it would extend across the company's messaging platforms.
Zuckerberg cited the risks for political dissidents in authoritarian regimes if communications are not encrypted in his post. He also mentioned a Facebook employee had been jailed in an unnamed country for not providing access to a user's private information.
"We've chosen not to build data centers in countries that have a track record of violating human rights like privacy or freedom of expression. If we build data centers and store sensitive data in these countries, rather than just caching non-sensitive data, it could make it easier for those governments to take people's information," Zuckerberg wrote. "Upholding this principle may mean that our services will get blocked in some countries, or that we won't be able to enter others anytime soon. That's a tradeoff we're willing to make. We do not believe storing people's data in some countries is a secure enough foundation to build such important internet infrastructure on."
That led some observers, including Facebook's former chief security officer, Alex Stamos, to interpret that the company was giving up on aspirations to gain a toehold in China, where the internet is censored by the government and Facebook and its apps are blocked unless users have access to a private internet network.
"Zuck has clearly given up on entering China, as these changes makes that impossible. Good." Stamos tweeted.
However, the company did not offer specifics on the countries in which it would not store data and denied it would not make a play for China.
"I don’t think you can take Mark’s post as any declaration against or for specific regions or countries," a Facebook spokesperson told ABC News in an email. "We’re in very early stages with realizing this vision, so anything beyond the letter at this point would be speculation. In general, our position on China has not changed. We have long said that we are interested in China, and our focus continues to be on helping Chinese businesses and developers expand to new markets outside China by using our ad platform."