In the last couple of years there's been an explosion of social messaging apps, of which WhatsApp was obviously the breakout hit. But one could easily suggest that in buying WhatsApp Facebook is just playing 'Whack a mole', with dozens of other bubbling up: last summer I went through Google Play and found 50 such apps with over 1m downloads.
The data all of these give is highly variable, though - downloads and user aren't the same thing and though many apps give user figures occasionally, 'users' often means 'anyone who ever downloaded our app' (which WhatsApp has complained about, making a point of giving MAUs) and many don't even tell you that. So it's interesting to look at Google Trends for some of the biggest names that have been floating around. (And yes, Google trends is indicative but far from authoritative).
First, compare WhatsApp, where we know the numbers, with a few of the bigger names.
(Comparing WhatsApp with Blackberry and BBM is also instructive.)
Now, keep Viber for scale (it reported 100m MAUs when Rakuten bought it in February) and add a couple of the names that have floated around as regional winners.
Now compare Nimbuzz, an Indian player, with WhatsApp in India.
Now, a couple of the US hits.
Small globally, but big in the USA.
The big gap in this, of course, is that we really can't use it to look at the really big contenders - Wechat is still mostly in China where Google Trends is useless, Kakao in Korea has the same problem, and Line is too generic a search term to tell us anything much.
I've argued elsewhere that the lock-ins Facebook enjoyed on the desktop are much weaker on mobile - that it's much easier to switch between services and to use several at once. But at the same time, it does appear that WhatsApp has much greater scale than the alternatives globally (unless there's a huge new app I've just not heard of yet, which, frankly, is entirely possible). Still, there's a lot of regional variation: WhatsApp is certainly not dominant in the USA, China, South Korea or Japan. And (having said you can't use Google Trends to look at Line), Indonesia shows a fascinating mix.
As I suggested here, perhaps part of the future is messaging within other apps, rather than lots of dedicated messaging apps.