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The Facebook will restrict posts on gun sales.Reuters

The $19 billion deal to sell WhatsApp to Facebook is one of the biggest in tech history, a sale that makes instant billionaires of WhatApp's co-founders Jan Koum and Brian Acton.

The monstrous amount of money is the largest sum paid to a start-up as part of an acquisition, which is six times what Google paid for Nest in January ($3.5 billion). The insane amount is also a whooping 19 times of what Facebook paid for Instagram two years ago ($1 Billion).

It is so much of money that people will need to reach beyond the business realm to understand it hypothetically, as the Sydney Morning Herald notes. Charles Kenny, Senior Fellow Center for global Development tweeted that Facebook's investment in WhatsApp messaging service is equal to the total annual lending by World Bank.

It is equivalent to almost four times Australia's current annual overseas aid budget and only $4 billion dollars less than the US foreign Aid in fiscal 2013.

WhatsApp has an active 450 million users per month, who send and receive over 50 billion messages per day. It employs about 50 people in Mountain View, California.

The deal will include $12 billion in Facebook stock, $4 billion in cash and $3 billion in restricted shares. Most of the money in shares is likely to go to Acton and Koum, while the rest of it likely to be for employees.

Koum had arrived in the US as an immigrant from the Ukraine at the age of 16. He grew up collecting food stamps, he said in a rare public appearance in January.

WhatsApp doesn't have advertising and instead makes money from an annual fee of $1 from users. The app allows users to chat and send pictures and videos back and forth with friends after connecting to the internet. It works like Facebook Messenger, BBM, Microsoft's Skype and Apple's iMessage.

Hinting on why Facebook would be interested in WhatsApp in the first place, Mark Zuckerberg told SMH that it was "the only widely used app we've ever seen that has a higher rate of engagement that Facdebook itself".