But surprisingly, it is not the fastest growing messaging app despite one million downloads a day and a fabulous valuation.
The one which is logging the fastest growth is Telegram -- about 5 million downloads a day.
Launched on August 14, 2013 for iPhone and on October 20, 2013 for Android, Telegram had only 100,000 daily active users four months back.
"4.95 million people signed up for Telegram today. Telegram is #1 most downloaded iPhone app in 48 countries," tweeted Telegram on Sunday.
But why it is becoming so popular?
The Telegram software differentiates itself from WhatsApp by stating on their website, "Unlike WhatsApp, Telegram is cloud-based and heavily encrypted. As a result, you can access your messages from several devices including desktops and share an unlimited number of photos, videos and documents. Thanks to our multi-data center infrastructure and encryption, Telegram is also faster and way more secure.
On top of that, Telegram is free and will stay free — no ads, no subscription fees, forever."
Telegram is financially supported by Pavel Durov who founded VK, a social media website known as Russia's Facebook, while the technological aspects are looked after by Nikolai Durov.
Telegram has been set up as a non-commercial organisation and the funding comes from Durov's donation.
The idea is that Telegram should therefore not be influenced by the interest of a person or company.
Why are people switching to Telegram?
A lot of people who migrated to Telegram from WhatsApp say they don't want Facebook to have even more data on them. Sure, another company would have it, but for most of them, that's acceptable.
They are fine with a company having access to my email, another company having access to their social network, another one to their chat history. But having one company having access to all of that seems to scare a lot of people.
Telegram is not the only encrypted messaging app seeing a rise in sign-ups following WhatsApp's acquisition by Facebook. Threema – which also touts encryption as a key feature – doubled its user base in 24 hours, with 200,000 new users paying 1.79 ( Rs 184) each for the App, TechCrunch reports.