Google’s (GOOG) Android still represents the biggest slice of the smartphone pie in the monthly report issued Thursday by Millennial Media, the largest independent mobile ad network.

But for the first time since 2010, when Android began dominating Millennial’s Mobile Mix reports, Apple’s (AAPL) share of smartphone ad impressions grew a bit in March (from 27% to 31%) and Android’s shrank a little (from 51% to 48%).

Millennial attributes the shift to the launch on Feb. 10 of the Verizon iPhone, which accounted for 8.2% of U.S. iPhone impressions in March.

Among the other findings in Millennial’s March report:

• Apple was the leading device manufacturer on its network, represented 32% of the top 15 manufacturers impression share in March, a 14% increase month over month
• iPhone impressions grew 17%
• iPad impressions grew 7%
• Overall iOS impressions grew 29% versus Android’s 23%
• iPad application revenue doubled, according to Millennial, pushing iOS back into the app revenue lead, with 47% of the market to Android’s 36% and RIM’s 7%

Also on Fortune.com:

• Millennial Media’s Mobile Mix: Android leads smartphone impressions
• Android’s rapid gains on the iPhone in ad impressions have come to a halt
• Android, iOS tied for smartphone ad impressions
• iPhone share of mobile ad requests dropped from 55% to 48% in August

[Follow Philip Elmer-DeWitt on Twitter @philiped]