• by Mark Walsh  @markfwal, July 1, 2010

The expansion of social media on mobile phones is no secret. In a June report, comScore identified social networking as the fastest-growing content category across both mobile browsers and applications. And then there are the 100 million people worldwide accessing Facebook on mobile devices.

The latest piece of evidence in that vein is a 50% rise in mobile ad campaigns that send users to multiple social networking sites in the last six months, according to the May metrics report released today by mobile ad network Millennial Media. Overall, 12% of campaigns direct people to social sites as a call to action within ads, a higher proportion than send users to m-commerce sites or store locator maps.

While mobile advertisers are increasingly trying to drive users to brand pages or promotions on social sites, click-to-call remains the most common type of action they encourage in ads at 36%, followed by signing up for a service (29%), submitting a form (28%), application download (24%) and watching a video (23%).

More than three-quarters (77%) of campaigns overall directed users either to a mobile site or a custom landing page. When it comes to ad formats, Millennial said spending on rich media units was up 85% as marketers that use the higher-impact units "doubled down, making sure it was a key component of their May campaigns."

Spending on mobile rich media ads more broadly will increase dramatically with the launch of the iAd platform today, for which Apple has said it has already taken in $60 million in ad buys from a group of initial sponsors that includes Nike, Nissan, Sears and Target. Whether competing mobile ad networks like Millennial and AdMob enjoy a halo effect from higher spending on the iAd -- or just the reverse -- remains to be seen.

Among other findings from the Millennial report, the number of ad requests per page view was just over one (1.03) and the average monthly page views per user was 108. The average session time was up by a minute to five minutes -- three seconds -- as the ending of the school year led to more time spent on mobile phones.

Millennial's audience grew slightly in April to 61.5 million from 60 million unique users, or 82% of U.S. mobile Web users.