Are Google's best days behind it?

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Shares of Google may struggle in coming years as the company faces a maturing search business, lower margins in other products and increased competition from Facebook, according to a Wall Street analyst Thursday.

"We do not see significant sources of future upside for shares and believe the best days for shares may be behind it," wrote Stifel's Scott Devitt, who downgraded the shares to "hold" from "buy."