WhatsApp, the global messaging app, announced that it will share users' phone numbers with its parent company, Facebook, in a move that critics say could eventually weaken the company's commitment to a high-privacy, ad-free experience.

Phone numbers provide valuable marketing information for Facebook, as the site does not require its own users to include a current phone number when making an account. This information would allow Facebook to better target ads for WhatsApp users and suggest friends based on whom they communicate with in the app. Facebook will not have access to messages' content, however, or be allowed to send ads in WhatsApp itself.

"This is a strong-arm tactic on the part of Facebook," Jeff Chester, executive director of the Center for Digital Democracy in Washington, D.C, told the Associated Press. "They continue on a campaign to run roughshod on our privacy rights."

Users will have 30 days to opt out of sharing their number with Facebook by unchecking a box, giving them some control over the data WhatsApp's parent company will collect. However, the move has still earned criticism from privacy advocates.

"Very few people opt out, it should be an informed opt in," Mr. Chester said. "No data should be used unless people are informed honestly about how it's going to be used."

WhatsApp has long made a commitment to user privacy. In April, the app announced that it would install default end-to-end encryption for its more than 1 billion users' messages, so that not even the WhatsApp team can see their contents – meaning it won't be able to comply with court orders to release them.

Facebook acquired the app in 2014 for $21.8 billion dollars, a particularly impressive sum considering that the app is free, and that its founders oppose in-app advertisements, wanting users to have a spam-free experience. Ever since, Facebook has been looking for a way to make money off of WhatsApp's 1 billion global users.

After the acquisition, WhatsApp chief executive officer Jan Koum assured users that it would not mean a step away from the company's values.

"If partnering with Facebook meant that we had to change our values, we wouldn't have done it," Mr. Koum wrote in a blog post. "Instead, we are forming a partnership that would allow us to continue operating independently and autonomously. Our fundamental values and beliefs will not change. Our principles will not change. Everything that has made WhatsApp the leader in personal messaging will still be in place."

Along with the announcement that it will share phone numbers with Facebook, WhatsApp also issued a revision of its terms of service for the first time since 2012. While the company will still bar traditional ads, it is looking into ways for business to communicate with customers on WhatsApp through confirmation notices, receipts, and package delivery tracking, and possibly messages about sales – all of which the company says users will be able to control in settings.

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"We do not want you to have a spammy experience," the company tells users in a summary of the new policies.

This report includes material from the Associated Press.