Shares of Google Inc fell as much as 4 percent on Friday, after a second straight quarterly slip in search advertising rates took the gloss off a handy first-quarter beat.

The world's top Web search engine attributed the 12 percent drop in its cost per click (CPC) for the first quarter to a shift to cheaper mobile advertising rates among other factors.

The company also announced a stock split designed to preserve the control of co-founders Larry Page and Sergey Brin over the world's No. 1 Web search engine.

Google's explanation for the declining CPC on a conference call with analysts offered clarity, but may not convince bearish investors, BMO Capital Market analyst Daniel Salmon wrote in a research note.

Mobile is increasingly an area of concern. Google has roughly 90 percent share of mobile search, but this revenue must be shared with OEM handset manufacturers and carriers, said Benchmark analyst Clayton Moran.

The fall in ad rates follows an 8 percent decline in the fourth quarter of 2011.

Barclays analyst Anthony DiClemente, however, said he was less concerned by this decline than others as cost-per-clicks was only part of Google's revenue growth prospects.

(We) are of the belief that CPCs will improve over time as Google's core search business will monetize well on mobile devices, he said in a note.

Analysts also said the lack of visibility around Google's plans for the recently acquired Motorola Mobility Holdings may pressure shares in the near term.

These results bode well and keep us positive on Google long-term, all the while recognizing that the lack of clarity around MMI is likely to remain an overhang on the stock short-term, Jefferies & Co analysts, led by Youssef Squali, said in a note to clients.

Shares of the company fell to $623.54 in early morning trade on the Nasdaq. They were trading down 3 percent at $631 later in the morning.

The stock, which fell 9 percent after Google's fourth-quarter results on Jan 19, has risen 2 percent since then.

(Reporting by Sayantani Ghosh in Bangalore; Editing by Saumyadeb Chakrabarty)