Shares of ValueClic, the Westlake Village-based Internet advertising company, fell to their lowest level in almost a year today after an analyst at Piper Jaffray Cos. downgraded the stock over concerns that Internet ad buying is shifting to online exchanges. The stock declined 7.2 percent to $14.47, making it one of the 10 biggest percentage losers on the NASDAQ at the close of trading. According to Bloomberg, analyst Mark Zgutowicz downgraded the shares to neutral because of what he said was an “acceleration of impression buying via online exchanges.” He said this is likely to lead advertising buyers to shift their purchases away from ad networks such as ValueClick, which provides online ad campaigns. Zgutowicz now expects ValueClick’s revenue to grow seven percent year over year for the rest of 2012, down from a previous estimate of 9 percent, according to Bloomberg. ValueClick reported a 31 percent increase in revenue to $152.8 million for the quarter ended March 31. Net income was up 28 percent to $21.6 million. The analysts noted that ValueClick itself participates in the online exchanges, but it’s competing with other buyers, and the profit margins on these exchange-bought ad impressions are lower than for ValueClick’s own networks. ValueClick could accept the lower margin business, in which case it is likely to miss projected earnings for the quarter, valuing the shares at $14. Bloomberg reported the analyst as concluding. If the company is able to offset the challenge, shares could be worth $17. Company officials did not comment on the slide in the shares. Judy Temes