By Molly Hensley-Clancy
NEW YORK, Feb 20 (Reuters) - What in the world is WhatsApp?
The online messaging platform has been catching on for a few years with younger users and international sets of friends, but a much larger audience noticed it on Wednesday, when Facebook Inc said it had agreed to pay $19 billion for the service.
It is a phone and mobile device app to send text, video and picture messages, and 450 million people use it monthly, some 200 million more than Twitter, according to numbers from the companies.
And it's adding a million users daily, which caught Facebook's eye.
WhatsApp works across different types of phones, across borders, and without ads. Unlike texts, there is no per-message charge, and there is no fee for international messages, which has helped make it popular outside the United States; WhatsApp charges a 99 cent annual subscription fee, which is waived for the first year.
Customers download the app, type in their phone number, and verify with a code sent by text.
While WhatsApp encrypts communication and does not store messages on its computers, satisfying some privacy concerns, it also prompts new users to allow the app access their address books, prompting another set of privacy issues.
WhatsApp faces a number of rivals, including Rakuten Inc's Viber, Naver Corp's Line and Tencent Holdings Ltd's WeChat. All are popular among young people, a demographic that Facebook has been losing out on in recent years and that are likely part of the draw for the company's purchase.
While kids may love the messaging, not all parents do. Mobile messaging is much more difficult for parents to monitor than Facebook, for instance, which has created new worries.
WhatsApp and Facebook both say the messaging service will maintain its independence once the deal goes through. (Editing by Peter Henderson)