I have not used Kik. I use WhatsApp. I would think the following would be reasons why WhatsApp scores over its competitors.<p>1. No need to log in - WhatsApp uses your phone number and a hash of your IMEI number as credentials to log in to their app. Although this means that your account could be compromised, its ease of use is therefore unparalleled.<p>2. No need to authorize people to add you - this means that if you have anyone's phone number, you already have them on your contact list. If I had to choose one reason for WhatsApp getting over a 100 million users in a short time, this would be it. A lot of people find this a privacy issue (including myself), but most people just don't care.<p>3. Sending images and video through WhatsApp is super easy and the content is compressed.<p>4. WhatsApp has restricted their emoticon set - I think this makes a big difference in keeping the clutter down.
Whatsapp is a "SMS / MMS" killer app, designed and especially <i>marketed</i> from the ground up to be one.<p>Facebook's Messenger is marketed as an IM, not as an SMS/MMS killer. Chat functionality is still seen as "bolted on" and unreliable for Facebook, so people turned to alternatives, and WhatsApp is by far the most recommended solution (peer pressure: if everyone has it, you want/need it too)
Facebook chat is slower and more unreliable. Reloading the chat on mobile after you sent some messages on another device is quite slow, messages also take 5+ seconds to arrive in mobile after they already arrived on PC, and this is on wifi.<p>The product just isnt as good.
In Hong Kong and mainland china whatsapp is pretty ubiquous,its shown on adverts and business cards as a contact method. Some mobile carriers make whatsapp data traffic free on most plans.<p>Also the big reason in mainland china is that Facebook is blocked there :)
Whatsapp doesn't care about a "friends list" or "approve friend request". If you have their number, you can do whatever you want to them. That's ease of use.