Amazon.com (AMZN.O) became the second most valuable publicly listed U.S. company on Tuesday, surpassing Google parent Alphabet Inc (GOOGL.O) for the first time.

Amazon shares finished up 2.69 per cent at US$1,586.51, for a market capitalization of US$768 billion, underscoring Wall Street's confidence in its relentless expansion into cloud computing, groceries and other new businesses.

Alphabet lost 0.39 percent, trimming its stock market value to US$762 billion, as Wall Street fretted about regulatory fallout following revelations that a political consulting firm had improperly obtained personal data on 50 million Facebook Inc (FB.O) users.

Together, Alphabet and Facebook dominate online advertising. They have previously faced government criticism for how they employ their user data.

Amazon's stock has surged 81 per cent over the past year, through Monday, bolstered by scorchingly fast revenue growth as more shopping moves online and businesses shift their computing operations to the cloud, where Amazon Web Services leads the market.

"They're using their cash flow to develop new businesses," said Tim Ghriskey, chief investment strategist at Inverness Counsel in New York. "They could have Apple in their sights at some point."

Seattle-based Amazon dislodged Microsoft Corp (MSFT.O) as the No. 3 U.S. company by market capitalization in February. Apple (AAPL.O) is the world's most valuable publicly listed company, with a market capitalization of US$889 billion.

Obviously, past stock gains are not a reliable predictor of future performance, and the surge in Amazon shares in recent years has been exceptional by most standards. But if Amazon's stock were to keep growing on the trajectory seen over the past year, its market capitalization would hit US$1 trillion in late August.

Apple's market cap would reach US$1 trillion around a month later if its stock price continued to rise at the 25 percent pace seen over the past year.

Alphabet's stock has risen 4 per cent so far in 2018 and is up 26 per cent in the past year.

The median of analyst price targets for the three companies put Amazon's market capitalization at US$823 billion, Alphabet's at US$914 billion and Apple's at US$989 billion, according to Thomson Reuters data.