WhatsApp is different to most messaging apps in that it runs on dumb phones (featurephones), not just smartphones. It's different to built-in text messaging in that it uses data rather than SMS. Dumb phones are a lot cheaper than smartphones, and for most people, data is cheaper than SMS.
There are an awful lot of people in the world who can't afford a smartphone or a serious SMS habit. Those people use WhatsApp.
There is a parallel here to BlackBerry Messenger. In the UK at least, the two groups that use (or used, a couple of years ago) BlackBerries are businesspeople, who use it because it integrates with their corporate IT whatnot, and schoolchildren, who use it because BBM is free, unlike SMS. However, BBM is tied to a shonky and still not that cheap hardware platform that uses special snowflake network services that are not available everywhere, whereas WhatsApp is portable.
That sounds rather trivial to me. Facebook has a huge userbase and I'm sure that if they made FB Messenger run on J2ME as well, it would have massive uptake. Seems like Zuck just wanted to spread the love around a bit and generate some news (or has some serious lack of confidence in the competence of his own developers).
Yes, everything about WhatsApp is so trivial that Facebook with their billions and billionaires and 1000's of high-quality engineers decided to spend 18 Billion dollars on it. But of course they're all dumbasses because you say it's all 'trivial', right?
I think it's pretty clear that Facebook didn't buy WhatsApp for the technology itself but rather the large market penetration/installed user base. Whether that was stupid or not is up for debate.
There are an awful lot of people in the world who can't afford a smartphone or a serious SMS habit. Those people use WhatsApp.
There is a parallel here to BlackBerry Messenger. In the UK at least, the two groups that use (or used, a couple of years ago) BlackBerries are businesspeople, who use it because it integrates with their corporate IT whatnot, and schoolchildren, who use it because BBM is free, unlike SMS. However, BBM is tied to a shonky and still not that cheap hardware platform that uses special snowflake network services that are not available everywhere, whereas WhatsApp is portable.