The decline comes after the investment weekly Barron's warned in its latest edition that the company faces fierce competition and that its share prices may be cut in half.
It said that click fraud, competition from rival internet giants Yahoo! and Microsoft, and doubts about the effectiveness of advertising on Google could all start to bite.
Google shares floated at $85 in August 2004, giving it an initial public offering value of $3.3bn. The shares rose to $411 on January 11 this year, but fell back to $345.70 at the close yesterday. It is now valued at $100bn.
Sentiment on Google has become much more negative since its fourth-quarter results last year disappointed because they fell short of analysts' forecasts, despite profits jumping by 82% year on year.
However, analysts remained bullish on Google yesterday, saying it is not overvalued, with target prices ranging from $490 to $540 a share based on confidence in Google's long-term prospects.
Analysts have pointed out that Google is still by far and away the leader in search marketing and that its business is growing rapidly.
Barron's said that Google's revenue forecasts might be over-optimistic and its advertising costs could be 5% too high.
The magazine also warned that Microsoft and Yahoo!'s search technologies were gaining on Google, while book publishers and newspaper publishers were likely to challenge Google over its move into their markets.
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