Mobile advertising company Millennial Media, which says its network reaches 83 percent of the mobile Web, said ad requests on Apple’s iOS rose 36 percent from May to June, when the company launched the iPhone 4.

The launch of the iPhone 4 appears to have strengthened Apple’s grip, which had shown signs of erosion by Google’s Android operating system.

Apple’s share of the smartphone market on Millennial’s network rose from 48 percent in May to 56 percent in June while Android’s smartphone market share fell from 15 percent in May to 11 percent in June. Apple has been the leading device manufacturer on Millenial’s U.S. network since September 2009.

Android ad requests rose, posting gains of 23 percent over May. Android’s ad requests have more than quadrupled since January.

BlackBerry maker Research in Motion (RIM) held its second spot among smartphones with 17 percent of the market, two percentage points lower than its May market share.

Across all devices, Motorola moved into fourth place among Millenial’s top 15 manufacturers, bumping down HTC and LG.