By Narottam Medhora

(Reuters) - Popular messaging service WhatsApp said it would start sharing users' phone numbers with parent Facebook Inc <FB.O>, marking a notable shift in its stance on privacy.

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

The change in policy, WhatsApp's first since the deal, will allow for more relevant advertisements and friend recommendations on Facebook, according to a WhatsApp blog post.

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

The company also maintained that messages on the service were encrypted by default and that it would not allow banner advertisements from third parties.

"Our belief in the value of private communications is unshakeable," WhatsApp said in the post.

Koum had outlined his approach to privacy in a blog post after the deal with Facebook, drawing on his own experiences of growing up in Ukraine during the Soviet era.

Some users, however, were not convinced by the shift in WhatsApp's stance.

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

WhatsApp said users could choose not to share account information with 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 on Thursday.

WhatsApp, which is 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. (

WhatsApp 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.

(Reporting by Narottam Medhora in Bengaluru; Editing by Saumyadeb Chakrabarty)