Are Internet users clicking on fewer Google ads and putting the company's growth prospects at risk?
Those questions are weighing on investors, who have cut the value of Google shares by 38 percent since they peaked at $747.24 in early November.
The slide continued Tuesday when Google shares dropped 4.6 percent to close at $464.19, as investors were spooked by a report by the research firm comScore that said clicks on Google ads the United States were flat in January when compared with a year earlier.
In all, Google's shares have fallen $283 from their peak, wiping out $83 billion in market value, and bringing a aura of vulnerability, at least on Wall Street, to a company that just four months ago seemed unstoppable.
Investors have focused with new intensity on Google's so-called paid clicks, which grew at 30 percent in the fourth quarter, because the search and advertising giant earns the vast majority of its revenue from text ads, for which it is paid only when users click on them.
Many analysts saw the comScore report as the clearest sign that Google, which does not give forecasts about its future performance, is not impervious to the slowdown that is buffeting the United States economy.