When Facebook bought WhatsApp, I couldn't help trying to recollect how I first started using the app and how it had surged out ahead of many of its competitors. I have an entire folder of just over a dozen chat apps on my phone, but I don't recall WhatsApp being the first of them I tried, or even the most beautifully designed. When I went searching for a free alternative to SMS, I recall trying to get folks on Textie, GroupMe, or TextPlus, among others, back in the day. Was WhatsApp the first mover? I don't recall that it was. Later, I joined WeChat, KakaoTalk, and Line to stay in touch with friends in Asia, and these were not just more beautiful in design but more sophisticated platforms than WhatsApp.

So how did WhatsApp break out? I don't know for sure, but I have a theory about one contributing factor.

While many have observed that the address book on your phone is a key social platform in itself, especially since apps can get access to it on iOS and Android phones, the phone number rather than just email address is a particularly unique slice of the contact book, the key of a social platform in and of itself, albeit the social network of a bygone generation.

Because WhatsApp uses your phone number as its user key, it can't be used across your devices. I can only use it on my phone. No tablet or desktop access. But what they lose in footprint across your devices they gained in ease of setup and speed of assembling a very high signal social graph. Instead of having to pick a username and password and remember those, all you had to do was enter your phone number and then wait for a text message to confirm that phone number belonged to you. Then you granted access to your phone's contact book and WhatsApp was off and running to create the largest possible copy of your social graph using phone numbers as the key for each user.

It just so happens that the people you have phone numbers for, the social graph of the previous generation, was and still might be perhaps the best proxy of the graph of people you are closest to. It's an even more intimate graph, in many ways, than your Facebook friends list, mine of which is bloated due to the generational norm of friending freely. The phone number is a holdover from an age when a phone call was an exciting thing to receive, not an annoyance as it is today, in this age of asynchronous communications, all that texting, emoji, and stickers.

Since most people tend to have phone numbers only for people who live in the same country, international calls being a rare occurrence given their high cost and the availability of other ways to reach people overseas now, WhatsApp likely grabbed quick local footholds and spread out within countries.

I've written before about how messaging doesn't have to be a winner-take-all space because of low switching costs. However, that doesn't mean there aren't network effects to messaging that allow one service to seize a lot of share in a local market, like WeChat in China or KakaoTalk in Korea or Line in Japan. WhatsApp has enormous share in Latin America, certain European countries, and Hong Kong (since many users are like me and use multiple chat apps, share of market of all messaging apps can add up to more than 100%).

More importantly, the ease of setup of using a phone number shouldn't be underestimated. TextPlus and GroupMe required a username and password, a more painful way of setting up one's account, especially on a phone. As way of research I went back and relaunched both of those apps for the first time in ages, and I got stuck on the login screen. What was my password for these services? I couldn't remember and couldn't be bothered to recover them. It's painful enough on a web page to try and recover one's password, it's even more so on the phone where password managers can't enter a password for you in another app.

It's not just painful remembering passwords on the phone, it's painful thinking of one. I've been trained, given all the horror stories of accounts being hacked, into letting my password manager think of some unique, long, and random password for every service that requires one. But that means they're no longer ones I can commit to memory. Given we live in the age of the smartphone, having a quick text sent to your phone with a short code and then keying that into an app to complete account setup is really quite painless as an alternative. Less secure? Probably, but most users discount that heavily.

Given the sheer volume of people going through setup, tiny edges in setup can have a multiplying effect that snowballs.

Someday when don't even have phone numbers anymore, when we just communicate via VOIP, the window for capitalizing on the the graph of people connected by phone numbers will close. In a crowded messaging space, at a time when many people were searching for an alternative to SMS, which was ripe for disruption given its criminal markup, the phone number may have been rocket fuel for WhatsApp's growth.

It's a theory, at least. I'd be curious to hear from others who were following the messaging space more closely in those days.

[UPDATE: Ben Thompson points out that WeChat, Line, and KakaoTalk all also use phone number as the primary method for creating an account, just like WhatsApp. I should have been clearer in my post above that I thought WhatsApp's simpler setup was only an advantage versus chat apps like TextPlus and GroupMe which did not have an option to sign up using a phone number. In Asia, many apps offer sign up through phone number, not just chat apps. It's just more of a convention in Asia.

This may be one reason WhatsApp did not come to seize dominant market share in some of the key Asian markets like China (WeChat), Japan (Line), and South Korea (KakaoTalk). Another is that WeChat, Line, and KakaoTalk are more sophisticated platforms for which chat is just one piece of the offering. They are also platforms for commerce, gaming, and virtual goods like stickers, none of which WhatsApp offer.]