Read the incredible story of Jan Koum. WhatsApp's co-founder from living on food stamps to been a billionaire.

When WhatsApp co-founder Jan Koum signed a deal with Facebook on Wednesday he became an overnight billionaire, but when he first moved to the U.S. he needed food stamps to survive. 

Koum, who is estimated to now be worth about $6.8 billion, was 16 when he moved to the U.S. from Ukraine with his mother. 

He had been raised in a rural community, in a house with no hot water or electricity, and when Koum moved to the U.S. his mother packed their suitcases with school supplies to save money.

But Koum's fortunes soon changed and on Wednesday he returned to the Mountain View welfare office, where he used to queue to get food stamps, to sign his historic deal with Facebook.

The offices for WhatsApp, which 37-year-old Koum created with Brian Acton in 2009, are also only a few blocks from the welfare office.

The pair stood outside the welfare building today as they signed the $19 billion deal with Facebook, only this time Koum was able to drive there in his Porsche.

His humble beginnings appear to have instilled in him a strong work ethic and dislike for egotism - WhatsApp may be a global phenomenon but it has no sign at its office.

'I can’t see a reason for there being a sign. It’s an ego boost,' he told Forbes. 'We all know where we work.'

Koum and Acton developed WhatsApp in coffee shops and at their homes. It took just a few years for the app to be worth billions of dollars. 

Facebook has paid $12 billion in stock and $4 billion in cash for WhatsApp, and the founders and employees will be granted $3 billion in restricted stock that will vest over four years after the $19 billion deal closes.

The deal shows how much life has changed for the developers, with co-founder Acton being turned down for a job with Facebook in 2009. 

In a tweet on his Twitter account at the time, he had posted: 'Facebook turned me down. It was a great opportunity to connect with some fantastic people. Looking forward to life's next adventure.'

It appears that Twitter turned Acton down for a job in the same year.

Facebook has said that it will keep WhatsApp as a separate service, just as it did with Instagram, which it bought for about $715.3 million.

WhatsApp has more than 450 million monthly active users. In comparison, Twitter had 241 million users at the end of 2014.

Facebook CEO Mark Zuckerberg says WhatsApp is on path to reach a billion users.

'The combination of WhatsApp and Facebook will allow us to connect many more people round the world,' Zuckerberg said.

'It's the only app we've ever seen that has grown more quickly than Facebook itself.'

'This is an incredible moment for me,' Koum said. 'Every day over 19 billion messages are sent, with over 1 million new users every day. We wanted it to be simple, and a better service than SMS.'

Facebook promised to keep the WhatsApp brand and service, and pledged a $1 billion cash break-up fee were the deal to fall through.

Zuckerberg said he was not planning to put ads on WhatsApp and Koum also pledged not to add adverts to the service, which makes money from a subscription model.

'We think advertising is not the way to go - we create a direct relationship with customers,' Koum said.

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