If you thought Facebook (META 2.67%) was the fastest-growing social media platform, you're wrong -- that feat would have to belong to WhatsApp, the multi-platform messaging service that Facebook announced it would acquire last week in a $19 billion deal (including the value of common shares and restricted stock units for employee compensation). At $19 billion for a 55-employee company, the app should be growing fast. But how fast, exactly, is WhatsApp growing? Arguably faster than any other social network in history -- even faster than Facebook was growing when it was WhatsApp's age.

WhatsApp isn't growing just slightly faster than Facebook... it's growing much faster. For instance, four years after launch, WhatsApp had about 420 million monthly active users. Compare that to Facebook's approximate user base of 150 million by that time.

In the video below Fool contributor Daniel Sparks takes a closer look at WhatsApp's size, growth, and a few other interesting facts about the messaging platform's scale.