Shares of Facebook at last check were down 1.5% to $273.79. Google slipped 2% to $1752.18.
Federal and state antitrust authorities are preparing to file new lawsuits against Google for abuse of its competitive position in search and advertising practices, people familiar with the matter told the Wall Street Journal. Facebook faces action regarding its social-media dominance, the paper reported.
Last month, the U.S. Department of Justice filed a long-awaited antitrust case against Google.
The DoJ said the action was brought to "restrain Google from unlawfully maintaining monopolies in the markets for general search services, search advertising, and general search text advertising in the United States through anticompetitive and exclusionary practices, and to remedy the effects of this conduct."
Last month, top executives of these internet companies faced tough questioning from a Senate committee about alleged anti-conservative bias.
Seattle online-retail and cloud-computing giant Amazon (AMZN) - Get Report will face a fresh antitrust battle with the European Union after the bloc filed formal charges against the company on Nov. 10.
Apple (AAPL) - Get Report is also facing two antitrust probes from the European Commission over its Apple Pay service and how it purportedly controls in-app purchases among companies that sell apps on its App Store.
Last year, EU antitrust regulators fined Alphabet $1.7 billion for unfairly restricting rivals from displaying ads on its platform via third parties using its AdSense tool.
Shares of Apple were up 1.2% to $118, while shares of Amazon were down 1.6% to $3,145.18.