Popular mobile messaging service WhatsApp has announced that it will drop its US$0.99 (S$1.42) annual subscription fee, and be made available for free from now on.The Facebook-owned service, which has a user base of 900 million users worldwide, is a messaging service for mobile devices, and can also be used to make voice calls over a phone’s Internet connection. It recently allowed users to access the mobile service on the Google Chrome web browser.