Global messaging service WhatsApp says it will start sharing the phone numbers of its users with Facebook, its parent company. That means WhatsApp users could soon start seeing more targeted ads on Facebook — although not on the messaging service itself, the Associated Press reports.

The move is a subtle but significant shift for WhatsApp, which has long promised to safeguard the privacy of more than 1 billion users around the world. WhatsApp is giving users a limited time to opt out of sharing their information with Facebook, although they must take the extra step of unchecking a box to do so. It also says Facebook won't post phone numbers online or give them out to anyone.

But the giant social network has been looking for ways to make money from WhatsApp since it bought the service two years ago, in an eye-popping deal ultimately worth $21.8 billion. At the same time, Facebook has pledged not to interfere with a longstanding promise by WhatsApp's co-founders to respect users' privacy and keep ads off its messaging platform.

WhatsApp on Thursday, August 25 offered a glimpse of its plans for turning on the money spigot, releasing new documents that describe the company's privacy policy and the terms of service that users must agree to follow. The documents are the first revision of those policies since 2012, before Facebook acquired WhatsApp, AP says.

One change follows through on previous hints by WhatsApp executives, who have said they're exploring ways for businesses to communicate with customers on WhatsApp. That could include using WhatsApp to provide receipts, confirm a reservation or update the status of a delivery.

Companies could also send marketing offers or messages about sales to individual customers, according to the new documents, which note that users will be able to control or block such messages. WhatsApp says it will continue to bar traditional display ads from its service.

Another change is potentially more controversial: WhatsApp says it will begin "coordinating" accounts with Facebook by sharing WhatsApp users' mobile phone numbers and device information, such as the type of operating system and other smartphone characteristics. The company says Facebook will employ the phone number internally to better identify WhatsApp users on Facebook, so it can recommend friends or show targeted advertising.

WhatsApp says current users have up to 30 days to accept the new policy terms or stop using the service. Once they accept, they have 30 more days to opt out of sharing with Facebook.