By Alexei Oreskovic SAN FRANCISCO (Reuters) – Google Incs first-quarter revenue fell short of Wall Street targets and margins narrowed as the price of its ads continued to decline, underscoring the challenges Internet companies face as the world shifts toward mobile devices. Shares of Google were down 3 percent to $539.80 in afterhours trading on Wednesday, after initially sliding roughly 6 percent on the news. The number of paid clicks by consumers on Googles ads increased by 26 percent in the first quarter, disappointing some analysts who had hoped for stronger volume growth. And the average cost per click declined 9 percent, extending a downward trend as mobile advertising, typically cheaper than traditional online ads, make up a bigger slice of its business.