Jeff Bezos and Mark Zuckerberg have each seen their fortunes swell by more than $20 billion this year

Jeff Bezos and Mark Zuckerberg, the founders and CEOs of Amazon.com and Facebook, respectively, have added a gob-smacking amount of money to their wealth in 2017 - more than $40 billion combined.

Bezos and Zuckerberg have each seen their fortunes swell by more than $20 billion this year as their companies' have thrived, according to data from the Bloomberg Billionaires Index.

Amazon's chief executive has gained $22.6 billion year-to-date and is now the second-richest person in the world with a net worth of $87.9 billion, trailing only Bill Gates ($91.7 billion).

Zuckerberg is up $20.1 billion on the year, and his $70.1 billion fortune makes him the fifth-richest person on earth.

The secret to their riches is pretty straightforward: Both of their net worths are tied heavily to tech giants they founded and run, each of which is having a strong performance in 2017.

For Bezos, 93% of his net worth, or $81.7 billion, is tied up in Amazon stock, dwarfing other high profile investments like rocket company Blue Origin ($3 billion) and The Washington Post ($250 million), according to Bloomberg data. Amazon stock is up 36% year-to-date.

Zuckerberg has nearly all of his fortune in Facebook stock as well - $67.6 billion, or 96%. The rest is comprised of cash, real estate, and other miscellaneous assets. Facebook stock is up 41% in 2017.