Mark Zuckerberg, Facebook's 29-year-old co-founder, lost $3.1bn in the five days to 28 March as bank and technology shares dropped on Wall Street.
The world's largest social network fell about 11% last week, after the company announced it would acquire virtual-reality firm Oculus VR for $2bn (£1.2bn, €1.4).
Zuckerberg is ranked 22nd in the global rich list with a $27bn fortune, according to the Bloomberg Billionaires Index.
Brin, Page, Bezos and Musk Lose
Meanwhile, Amazon.com CEO Jeff Bezos's fortune fell $1.9bn after the world's largest online retailer said it would cut prices for cloud services, a day after Google cut its prices.
Google co-founders Sergey Brin and Larry Page lost a combined $3bn as the world's largest Internet search firm prepared to issue 330 million nonvoting C Class shares this week, to fortify the founders' control over the company.
New prices for cloud services from both firms would take effect on 1 April.
Tesla Motors boss Elon Musk , whose fortune surged to $12bn in late February, lost $634m last week, reported Bloomberg, as the electric carmaker's stock dropped 7%.
Tesla said it would add new battery shields to its Model S electric-powered sedans. US regulators have ended a four-month investigation into fires that occurred after the cars struck road debris.
Larry Ellison Gains
Oracle's 69-year-old founder Larry Ellison added $1.7bn to his net worth, making him last week's biggest gainer. Shares of the world's largest database company advanced 5.5%, its biggest weekly jump since December 2013.
Ellison is ranked 8th in the world with a $44.3bn fortune.
The technology-heavy Nasdaq Composite finished 2.8% lower at 4,155.76 points for the trading week ended 28 March.
The Standard & Poor 500 finished 0.5% lower at 1,857.62 points for the week ended 28 March.
Bill Gates remains the world's richest person with a net worth of $79bn. Gates is followed by Mexico's Carlos Slim, who holds a $65.5bn fortune and Spain's Amancio Ortega, who has a net worth of $63.8bn.