Facebook turned down WhatsApp co-founder Brian Acton for job in 2009

It turned out to be an expensive mistake for Mark Zuckerberg. A $19bn (£14bn) one to be precise.

In the summer of 2009 Facebook turned down WhatsApp co-founder Brian Acton for a job.

Like any other dejected interviewee, he used Twitter to express his glass half full disappointment: "Facebook turned me down … looking forward to life's next adventure."

That adventure involved Acton and partner Jan Koum, a Ukrainian immigrant whose childhood experience of Soviet era surveillance inspired the WhatsApp messaging service. Now a blockbuster deal has turned them into multibillionaires.

In poignant nod to Koum's rags to riches success, the deal was signed in a now empty office block where his family once collected their food stamps.

The men are an unlikely double act. While Koum survived a tough schooling in a village outside Kiev and dropped out of San Jose State University, Acton, a Stanford computer science graduate, whiled away his time playing golf in suburban Florida. Their paths crossed at Yahoo in the noughties and in 2009, two years after they had left, WhatsApp was launched.

The thinking behind WhatsApp is rooted in Koum's memories of a country where phones were tapped and school friends were censured for their views. He told one interviewer: "I grew up in a society where everything you did was eavesdropped on, recorded, snitched on. Nobody should have the right to eavesdrop, or you become a totalitarian state – the kind of state I escaped as a kid to come to this country where you have democracy and freedom of speech. Our goal is to protect it."

Though Koum is only in his late 30s and Acton in his early 40s, the men are older than many of their Silicon Valley brethren: Zuckerberg is only 29. But Acton says age brings perspective as investors salivate over fast-growing services such as Snapchat: "Great, teenagers [on Snapchat] can use it to get laid all day long," he told one interviewer. "I don't care. I'm 42, essentially married with a kid. I don't give a shit about this. I'm not sexting with random strangers."

At 6ft 2in, Koum, who moved to California with his mother and grandmother in 1992, favours the tech start-up uniform of T-shirt and hoody and holds an apparently healthy attitude to swearing. He started to study maths and computer science but confesses to having been "equally bad at both". He favours a low profile, providing a Twitter biography limited to the Kanye West quote "We on a galaxy the haters cannot visit" to sum up his views. His LinkedIn account is also testament to a droll sense of humour with a minimal CV. He says he "barely graduated" from high school and "dropped out" from university. At Yahoo, he "did some stuff". His entry actually says "see Brian's profile for more details".

Acton's exhaustive CV on the networking site is evidence that he is happy to play the straight guy. He speaks nine languages, but they don't help on holidays as they are the "C++, Perl, PHP, Erlang, Java, Python, JavaScript, ActionScript" codes. He tempers details of his impressive credentials with a well rounded list of hobbies that includes ultimate frisbee, snowboarding and watching movies.

Acton describes the relationship as "yin and yang". "I'm the naive optimist, he's more paranoid. I pay attention to bills and taxes, he pays attention to our product."

On Wednesday, Koum, whose father stayed behind when they left Ukraine, used the WhatsApp blog to reassure users they hadn't sold out. He wrote: "Our team has always believed that neither cost and distance should ever prevent people from connecting with their friends and loved ones, and won't rest until everyone, everywhere is empowered with that opportunity."