Data from the AdMob Mobile Metrics Report shows that network ad impressions increased 1.6 percent in April compared with March, but requests were down 0.4 percent.

AdMob Inc.'s mobile ad network generated 2,595,092,049 impressions for April versus 2,553,018,899 for March. Requests for April were 2,854,226,146 versus 2,865,167,969 for March.

"Regarding patterns in mobile advertising, we see growth, sustained click-through rates and advertiser appetite for reach, as evidenced by fill rate at country and worldwide levels," said Jason Spero, vice president of marketing at AdMob, San Mateo, CA.

Twenty-five percent of ad requests in the AdMob network worldwide were from smartphones.

The company claimed U.S. requests grew 5.1 percent April over March to outpace network growth.
Motorola's Razr V3 and the Nokia N70 remained the top mobile device choices in the United States and India.

Similarly, the Sony Ericsson K800i maintained its top device status in Britain in April as did the Nokia 6600 in Indonesia and the Motorola v360 in South Africa.

The report found that the Amoi 8512, or the Three Skype phone, made its debut as the No. 5 mobile device of choice in Britain with 2.7 percent of that country's traffic. This was only on one operator -- Skype.

Also, the Apple iPhone made it to AdMob's list of top 20 devices worldwide.

AdMob said iPhone traffic continues to grow within its network as more phones make it to the market and more content developed just for that handset.

Data shows that many publishers in the AdMob network have built iPhone-specific sites that are seeing more traffic. AdMob serves more than 20 million ad impressions a month to the iPhone and more than 10 million ad impressions to the Apple iPod touch.

In an interesting comparison of traffic, AdMob found that Wi-Fi usage accounted for 54 percent of iPhone traffic in its network. It means that more iPhone users are surfing the Web on Wi-Fi than using AT&T's EDGE wireless network.

The iPhone is sold exclusively through AT&T in the U.S.

AdMob is monitoring this iPhone trend, especially as Wi-Fi coverage in the U.S. improves and more dual-radio devices launch in the market.

Given these market trends, Mr. Spero advises advertisers to experiment.

"This medium is evolving rapidly and advertisers are investing to understand how to optimize targeting, creative and landing pages," Mr. Spero said.

"Advertisers know their target customer," he said. "That is consistent across media. Focus on whom you want to reach.

"There are a ton of innovative built-for-mobile sites that can provide reach. These are providing reach and engagement for advertisers beyond what known brands can deliver. So, experiment with a mix of branded sites and new mobile brands."