WhatsApp says it will begin sharing more data with Facebook and will start letting some companies send messages to users.
WhatsApp will now share users' phone numbers with the social network, which it will use to provide "more relevant" friend suggestions and advertisements.
One analyst said some people might feel "betrayed" by the move.
WhatsApp said sharing users' phone numbers with Facebook would help tackle spam and abuse, as well as offer people "better friend suggestions and more relevant ads".
Using the data, Facebook will be able to match people who have exchanged phone numbers, but have not added one another as "friends" on the social network.
WhatsApp will also share information about when people last used the service, but said it would not share the contents of messages, which are encrypted.
"Your encrypted messages stay private and no-one else can read them. Not WhatsApp, not Facebook, nor anyone else," the company said in a blogpost.
The company said users would be able to opt out of sharing information with Facebook by following the steps outlined on its website.
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