Mark Gurman, Bloomberg:
Right now, when iPhone users ask Siri to call or message a friend, the system defaults to Apple’s Phone or iMessage apps. If you want to use WhatsApp or Skype, you have to specifically say that.
When the software refresh kicks in, Siri will default to the apps that people use frequently to communicate with their contacts. For example, if an iPhone user always messages another person via WhatsApp, Siri will automatically launch WhatsApp, rather than iMessage.
The article also contains a response from Apple on accusations that App Store search is tilted to prefer Apple apps. As the article says, feels like Apple is slowly loosening the reins.