American billionaire Sergey Brin loses $29 billion as Alphabet suffers its worst day since going public

American billionaire Sergey Brin lost about $29 billion on paper on Thursday, as Alphabet shares suffered their worst single-day drop since the company went public in 2004.

Alphabet shares fell more than 12% to about $283.29, from $321.89, after investors reacted to slowing growth in its core search business and heavy spending on artificial intelligence, Yahoo Finance reported. The Associated Press put the decline at about 13.4%.

The selloff wiped more than $500 billion off Alphabet’s market value, according to Bloomberg. Brin’s fellow Google cofounder, Larry Page, lost about $31 billion, bringing the founders’ combined paper losses to about $60 billion, based on Forbes estimates. Neither has confirmed selling shares.

Search was at the center of the selloff. Revenue from Google’s search business rose 12% to about $63.4 billion, short of the roughly 15% growth investors had expected. Search is Alphabet’s biggest source of revenue, and analysts see generative AI tools as a long-term threat to it.

Spending added to the worries. Alphabet’s capital expenditures reached about $51.4 billion in the quarter, roughly 40% of revenue, as it pours money into data centers and AI. The company has said it expects spending to “increase significantly” next year.

Strong cloud growth was not enough to calm investors. Google Cloud revenue more than doubled to about $30.7 billion, but the gain did little to offset concerns about search and cash flow.

Alphabet reported quarterly revenue of $127.6 billion on Wednesday, slightly below expectations, with earnings per share weighed down by an $8.7 billion loss on equity securities. Net income came to about $26.2 billion.

The drop is a sharp reversal for Brin, whose fortune is tied largely to his Alphabet shares. AI enthusiasm had lifted the stock for much of the year, pushing Brin and Page into the top tier of the world’s richest people.

Regulation is also weighing on sentiment. New European restrictions on how Microsoft, Amazon and Google handle certain sensitive data have added to the pressure on the company.

The losses come days before California voters decide on Proposition 40, a proposed one-time tax on billionaires’ fortunes. Brin is leading a $317 million campaign against the measure, which would be calculated on net worth, including unrealized gains on stock.

The losses are unrealized, meaning they exist only on paper unless the shares are sold. A recovery in Alphabet’s stock would restore much of Brin’s wealth, but Thursday’s drop shows how quickly AI-driven fortunes can shift.