Apple and Android continue to duke it out for the top spot on Millennial Media's ad network.

Each of the two competing platforms captured 38 percent of all smartphone ad impressions (how often an ad appears) in the U.S. for November, according to Millennial Media's "November Mobile Mix Report." That number reflects a 1 percentage point gain in ad impressions for both Apple and Android from October's results following Android's 6 percentage point gain and Apple's 9 percentage point drop from September.

BlackBerry maker Research In Motion took third place on Millennial Media's network with 19 percent of all ad impressions, leaving Microsoft's Windows Mobile OS and Nokia's Symbian each with a 2 percent share.

Looking strictly at mobile-device manufacturers, Apple kept its top spot on the ad network with the iPhone, iPod Touch, and iPad collectively capturing 25 percent of all ad impressions. Grabbing 17 percent of all impressions in November, Samsung held onto second place with its mixture of feature phones and Android smartphones. Motorola took home a 15 percent share, helped by its Droid 2 and Droid X phones. And with five BlackBerry devices on Millennial Media's top 30 mobile device list, RIM won 11 percent of all ad impressions last month.

Overall, smartphones accounted for 19 of the top 30 mobile devices in November.

Android mobile apps grabbed 54 percent of all ad impressions seen among apps on the Millennial Media network, averaging 10 percent monthly growth over the past four months. Apps created for Apple's iOS accounted for 38 percent. Gaming apps proved the most popular category in general, winning 28 percent of all ad impressions, with Music and Entertainment in second place with a 22 percent share.

Millennial Media currently covers and tracks ads on 85 percent of all mobile devices in the United States.