In four out of five activities, smartphones scored between 4-9 percent higher than PCs in terms of engagement satisfaction, according to Millennial Media?s August Smart report.

The shift from PC to mobile is clearly underway, as browsing the Internet ranked only 2 percent higher in engagement satisfaction on a computer than on a smartphone. With that in mind, marketers should also know that the United States' mobile Web traffic grew 3.12 percent, reaching 63 million users.

?One of the key takeaways from the August Smart is that summer trends continued,? said Mack McKelvey, vice president of marketing at Millennial Media, Baltimore, MD. ?One of the signs that mobile has arrived as a viable ad channel is that we are seeing seasonality across the network."

The report from Millennial Media explained that a huge trend for this summer was that advertisers leveraged targeting methods with the broadest reach.

Of the 63 million mobile Web users in the U.S., Millennial Media?s potential unique audience reach increased to 49.6 million, extending to an estimated 79 percent of the U.S. mobile Web.

Advertisers who used run-of-network tactics grew 18 percent and run-of-network, channel and custom subnet comprised 82 percent of the campaign targeting mix from the report.

Traffic-to-site represented 57 percent of campaign destinations actions, according to the SMART report.

While more users are surfing the mobile Web, the average Web session time among users dropped from five minutes, 10 seconds to four minutes, 59 seconds.

However, the Smart report found that average monthly page views per user increased to 111.

?We are noting that, although the addressable universe of mobile ad users are growing, their session times are hovering just under the five-minute mark,? Ms. McKelvey said. ?However, their page views are continuing to increase.

?This puts the pressure on the advertisers and their mobile ad network partners, to ensure that the creative and overall consumer experience is engaging and compelling, targeted correctly and optimized accordingly,? she said.

Fifty-four percent of the campaigns tracked by Millennial Media used frequency capping, hinting that it might be a growing trend in the coming months.

In the battle for the top-ranked device, Apple?s iPhone continues to lead with a 12.51 percent impression share.

While Samsung has been on top for a while as the top device manufacturer, Apple was No. 2 and closed the gap with Samsung a bit more.

The August report shows that iPhone and iPod touch impressions increased 15 percent, following two huge spikes in June (68 percent increase) and July (29 percent increase).

?Advertising Week has been extraordinary this year,? Ms. McKelvey said. ?Agencies and brands alike have been talking about mobile in a much more open and positive manner.

"Yet, one of the themes from Mobile Day is that agencies feel that there isn?t enough data available to understand the trends and opportunities in mobile,? she said. ?That?s what SMART set out to do.?