Google showed modest growth during February in a closely watched report released on Wednesday on how search sites like Google get paid by advertisers, and its shares fell 3.2 percent.

Web measurement firm comScore Inc issued a monthly report on "paid clicks" -- a key measure of how Web searchers are converted into ad viewers -- that showed Google's paid click rate grew 3.1 percent year-to-year in February, sources said.

Flat year-over-year growth by Google on the same basis in comScore's January report, which was released in late February, led to an 8 percent one-day drop in Google's stock the day afterward and a fierce Wall Street debate on whether revenue growth is slowing or if Google's own actions are to blame.

Lurking behind this discussion is investor concern that Google's growth in terms of paid clicks has slowed from around 30 percent to 40 percent six months ago, raising the prospect that the economy is to blame for some of Google's slowdown.

"You can't avoid the trend. Something that was humming along at 40 percent is now near zero: Something is going on there. Especially when the economy is getting a little twitchy," said RBC Capital analyst Ross Sandler. Last week he cut his 12-month price target on Google to a Street-low-level of $530.

ComScore declined to comment on the report, saying it publishes the data for use by clients, which include companies in the advertising industry and Wall Street brokerages, which use the statistics to gauge Google's quarterly revenue trends.

Shares of Google fell $14.52 to $443.67 in extended trade on Wednesday after the release of comScore's latest click data. The fall erased a $7.41 gain in the Nasdaq regular session.

Complicating the issue is Google's efforts in recent quarters to improve the effectiveness of advertising shown in the margins of its Web search services by paring back the number of ads per page and reduce inadvertent customer clicks.

"It is not as bad as comScore data shows it but it's not as good as Wall Street expects it to be," Sandler said.

"Clearly if you believe this data, then things are slowing (for Google)," UBS analyst Ben Schachter said, adding that, if paid click growth has flattened, then the only way to sustain Wall Street revenue expectations is for Google to charge more for the ads it sells, which may be tough as the economy slows.

Most Wall Street analysts expressed bafflement at what the data actually means, but said the slower trend is worrisome.

"We are not quite sure what it means. Overall the report says paid click growth is down. But in some of Google's operating metrics, there is an interesting improvement," Sanford C. Bernstein analyst Jeffrey Lindsay said. "It is premature to say that Google's revenue performance has been impacted by the economy or anything like that," he said.

One measure of how Google is paring back ads is that comScore data shows 14 percent reduction in the year-over-year average number of advertisements per search result page.

"While (Google)'s paid click data does little to calm investor fears of a slowdown in (Google)'s core business, we believe most of the deceleration is due to the continuing quality initiatives by the company itself," Banc of America analyst Brian Pitz said in a note to clients on Wednesday.

Adding to the confusion for investors are competing statistics from other sources that show Google is performing solidly by other measures of its online ad performance.

Web search marketing firm SearchIgnite, which places $300 million in ads on behalf of large- and mid-sized advertisers and their ad agencies, says its own data shows Google's year-to-year paid click rate growing 29.0 percent in February.

Roger Barnette, president of Atlanta-based SearchIgnite, said the amount of advertising spending by its clients on Google rose 37.9 percent in February from a year ago.

"The comScore data has been a bit more conservative regarding what they are tracking on Google. We just haven't seen that with our clients. We see significant year over year increases in clicks and media spending," Barnette said.

However, Barnette adds that, "Google is dealing with a lot of macro things which they weren't dealing with a year ago."

SearchIgnite's data is based on spending patterns of its 500 advertising clients. Google attracts nearly three-quarters of SearchIgnite client spending, Yahoo 20 percent and Microsoft draws 6.7 percent. ComScore monitors a panel of 2 million Web users worldwide.