WhatsApp is one of the most popular messaging apps. In the early days, BBM (Blackberry Messenger) was used by everyone but was limited to Blackberry devices. WhatsApp provided a free messaging platform for Symbian (still popular in emerging markets), Blackberry, Android, Windows Phone and iOS. It was one of the first apps to be truly cross-platform, not just iOS and Android. With Symbian and Blackberry widely used in emerging markets, it got a lot of market share. Dozens of messaging applications have emerged since then, but there are two important aspects to such an application: (1) mass adoption by networks of people, and (2) being truly cross-platform (not just Android and iOS).
When using a utility application for Mail or IM, its easy to switch to a better application. With messaging, it does not make sense to switch when five of your contacts are using it versus hundreds on the other platform. So people are not likely to switch. What developers also forget is that Android and iOS are the major OSes, but only in North America. Asian and South American markets are still dominated by Symbian and Blackberry devices with Android gaining market share rapidly. Also when messaging patterns for WhatsApp is analyzed, its heavily used for international communication. When all of you contacts are in the US, you could use texting as most carriers now offer unlimited free texting. But for international messaging, apps like WhatsApp are used primarily. This means that supporting platforms in those markets is as important as iOS support.
WhatsApp has introduced a lot of features over the years, including ability to send and receive media (images and videos) and groups. This feature set is transforming WhatsApp into a mobile social network. While Facebook is defined as the largest social network, most users are mobile in todays age. They also have all their contacts on their phones, most on WhatsApp too. This has changed user behavior to create groups in WhatsApp (similar to Google+ circles) and have a stream of images and videos shared in groups (similar to News Feed on Facebook). Everyone is in multiple groups and all members regularly post messages, images and videos in the group. This unintended behavior is increasing getting common. It has transformed WhatsApp into a social network. While a lot of usage on WhatsApp is still for messaging, groups and media sharing is increasing at a rapid pace. Specially in emerging markets where high speed internet and access to computers is limited, people with Symbian phones and a decent 3G connection can share and consume content on the go. Facebook also offers all of the services via the Facebook app and Facebook Messenger, but Facebook is viewed as a social network and the inability to easily broadcast content to a selected group of people (they have lists but very few know about it) is discouraging to users. This makes WhatsApp a direct competitor to Facebook and makes its acquisition a smart move.
This highlights a few key lessons. First is that platforms in emerging markets matter as a lot of messaging activity is international. Second is that the extensive use of WhatsApp as a social network highlights the need for a good social network centered around mobile usage. It could also mean not being on the desktop at all. And having the ability to just distribute content to groups rather than all contacts (such as all Facebook friends). With WhatsApp adding even more features such as Audio calls, it would be interesting to see how usage of the app evolves.