Android's impressions grew 39% month over month, while the iPad's grew 76%

    The news for Apple (AAPL) was mixed in the August report from Millennial Media, one of the larger independent mobile advertising networks.

    The iPhone, which once dominated the network's smartphone category with a 70% share, continues to drift downward, from 55% in July to 48% in August. The share of ad impressions on smartphones running Google's (GOOG) Android OS, meanwhile, reached 26%, up from 19% last month. Research in Motion's (RIMM) BlackBerry also had a good month; its ad share rose 3 points, to 19% from 16%.

    But the big winner last month was the iPad, whose share of ad traffic rose 76% between July and August, according to Millennial.

    Of course, these share changes are occurring in a broader market that itself is growing rapidly. According to IDC data released Monday, revenue from the U.S. mobile ad market is expected to more than double this year and could reach $500 million by December.