This question originally appeared on Quora. 
Answer by Kartik Ayyar, founder and CTO of a startup that will be launching in 2014, previously the CTO of Cityville at Zynga
I don't use Whatsapp, though I experienced a huge aha moment about recognizing how powerful it was when I traveled to India after a gap of 3 years.
I had previously visited India in 2010, and then made a recent visit in 2013. It was shocking to see how popular it was, especially since smartphone penetration was far less and nobody talked about it during my past visit.
More importantly, not only was it popular, it was used in India the way Facebook is used in the US, namely as the defacto sharing application of choice.
It is not uncommon to hear phrases in India that go:
- "Can you share it on a group in Whatsapp."
- "Can you send me that persons number on Whatsapp."
- "Can you install Whatsapp so I can stay in touch with you."
Furthermore, these are things that you hear from people who don't have fancy phones and are not well versed with technology - they were able to use the product knowing how to use a cell phone. At the other end of the spectrum, even users of powerful smartphones will use Whatsapp over Facebook simply because they have more friends on Whatsapp and it is simpler to use.
You can read a lot of the answers here, though to truly appreciate some of them fully, I'd say nothing drives the point home like actually traveling to parts of the world where smartphones are a primary computing device and Whatsapp dominates to see how powerful of a Facebook substitute it can be.