Two and a half years after being acquired by Facebook, private messaging app WhatsApp has announced that it wants to share your phone number with the world's biggest social media network. 

When the buyout was announced in 2014, WhatsApp founder Jan Koum vowed to protect data of its users, saying the deal would not affect its privacy policy.

But in a blog post announcing changes to its terms and conditions, WhatsApp said the company would be sharing information to allow it to better fight spam and lead to users getting better friend suggestions over Facebook.

The shared data will also help WhatsApp track information about how often people use its services and tackle spam on the service, it said. 

Facebook benefits by adding even more information about its users to its massive database, allowing it to sell more targeted ads.

WhatsApp sought to reassure users by saying that it would not sell, share, or give users' phone numbers to advertisers. 

But the backlash from many users and privacy advocates was immediate. 

"Phone numbers?!? No! That's absolutely NOT OKAY. I might need to delete Facebook, people. NOT kidding," Twitter user Mindy McAdams wrote.

Others began to openly discuss which messaging app to switch to, with many Twitter users suggesting Telegram as the best alternative. 

Protests have not been limited to irate users. The Washington-based Electronic Privacy Information Center (Epic) has accused Facebook of violating a 2012 agreement which required the company to obtain users' consent before changing privacy settings. 

"It's absurd that a company can disregard a legal judgement," Epic's executive director Marc Rotenberg told the Guardian.

WhatsApp, which has more than 1 billion users, will also explore ways for businesses to send messages using its platform over the next several months, it said.

The messaging app dropped its $1 token fee for some of its users earlier this year and said it was experimenting making businesses pay to reach their customers through the service.

How to keep your phone number private

The silver lining for privacy advocates is that users can choose not to share account information with Facebook. 

Users presented with WhatsApp's updated terms and conditions can click 'read' and then uncheck the box asking whether they allow the app to share their phone number with Facebook. 

Users who don't uncheck the box have 30 days to change their minds. To change the setting, visit Settings > Account > Share my account info and toggle the control. 

Full details on how to disable the setting can be found here

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