WhatsApp, the cross-platform instant messaging service that has been downloaded by almost everyone who has a smartphone, has been bought by Facebook for a whopping $23.9 billion.

Billions of people worldwide use the service regularly and and millions more download the app daily.

Here are 10 things you might not have known about WhatsApp:

1. WhatsApp Inc. was founded in 2009 by American Brian Acton and Ukrainian Jan Koum, both former employees of Yahoo!.

2. WhatsApp has more than 450 million users, 70 per cent of whom are active on a given day.

3. WhatsApp handles more than 10 billion messages and about 400 million shared photos daily.

4. WhatsApp adds more than one million new registered users per day. Its user growth in its first four years far exceeded the rates experienced by Facebook, Skype and Twitter.

5. WhatsApp has just 50 employees, most of whom are engineers. The company is based in Mountain View, California.

6. On Dec 31, 2013, WhatsApp reached a new daily record of 18 billion messages processed in one day.

7. One check on a message means it has been successfully delivered to the server. Two means the message has been successfully delivered to the phone of your chat partner. It does not mean that the message was read.

8. WhatsApp doesn't show advertisements or connect users to game platforms, but it stays profitable by charging a US$0.99 (S$1.25) annual subscription after one free year.

9. On why they don't have ads, creators Brian Acton and Jan Koum said: "... we wanted to make something that wasn't just another ad clearinghouse. We wanted to spend our time building a service people wanted to use because it worked and saved them money and made their lives better in a small way."

10. They described advertising as a "disruption of aesthetics", an "insult to intelligence" and an "interruption" of a users train of thought.