The fall has wiped about $420 billion off the company's value.
Shares in Alphabet, the parent company of Google, were down 12% on Thursday, after the company reported a slowdown in its core Search advertising business.
The fall has erased about $420 billion of market value and dragged the Nasdaq down 1.2%.
Alphabet remains America's fourth-largest company, with shares still worth more than $3.4 trillion in total.
The fall followed Wednesday's third-quarter results, which showed revenue below forecasts and weaker advertising sales.
Search and other revenue rose 12% to $63.4 billion, its slowest growth in five quarters and below the $64.9 billion analysts expected.
JPMorgan Chase analysts called the slowdown in Search "steeper than expected" in a note on Thursday morning.
Philipp Schindler, Alphabet's senior vice president, told investors fewer ads were appearing because of AI Mode, Google's AI-powered search feature, though he said ad experiments in the feature had shown promising results.
Earnings per share were $2.12 against the $2.99 expected, after an $8.2 billion loss on securities holdings compared with a $12.8 billion gain a year earlier.
Operating income of $40.6 billion missed the $43.2 billion forecast, as shared AI research costs rose to $7.7 billion from $4.5 billion.
Spending worries
Capital expenditure rose 114% to $51.4 billion, more than double the roughly $24 billion a year earlier, and free cash flow was negative $9.1 billion.
Alphabet's finance chief warned that capital spending would rise significantly in 2027, and the company has committed to more than $1 trillion in multi-year agreements for infrastructure, energy and leases.
The sell-off dragged the Nasdaq Composite down 1.6% and spread to chipmakers and equipment suppliers, while Caterpillar shares fell 9%.
Cloud bright spot
Google Cloud revenue jumped 103% to $30.7 billion, well above the $29.1 billion forecast, with an operating margin of 36.2% against 32.5% expected.
Executives pointed to more than 1 billion users of the Gemini AI app to offset the disappointment in Search.
Broker view
Yorkville Ives, the broker, kept its "outperform" rating and $400 price target, set against a share price of $284.
Analyst Dan Ives called the result "the Cloud print we have been waiting for" and said it mattered more than the softer Search figure.
He reads slower Search growth as monetisation lagging while AI Mode scales, not cannibalisation.
