According to a recent modification in WhatsApp's terms of service, the chat app will now share more of its users' personal data with its parent company, Facebook. One of the main things will be the sharing of the phone numbers database, which will allow Facebook to match each user's social network account with their WhatsApp account. Additionally, the two services will also trade data that will allow them to serve better targeted advertisements.
In the blog post that announced this change, WhatsApp's spokesperson wrote: "[B]y connecting your phone number with Facebook's systems, Facebook can offer better friend suggestions and show you more relevant ads if you have an account with them." While I understand that Facebook spent $19 billion for WhatsApp, so it can basically do whatever it wants, this still seems a bit abusive.
In case you're a fan of the service, you might also enjoy reading about these "6 WhatsApp features you probably didn't know about".