Android is now running neck and neck with Apple's iOS as the largest U.S. smartphone platform on the Millennial Media mobile ad network.

The number of mobile ad impressions (how often an ad appears) in the U.S. for Android jumped 8 percent in October over September, according to Millennial Media's "October Mobile Mix Report." That increase means both operating systems accounted for a 37 percent share of all impressions tracked by the ad network.

Ad requests (the number of times a mobile ad is clicked on) rose for both operating systems last month but gave Android, a later arrival on the scene, the more impressive gain. Requests on Android devices jumped 65 percent over September and have grown 2,182 percent since January. Requests on Apple's iOS devices rose 12 percent over the prior month and are up 32 percent since the start of the year.

Though Millennial Media is only one ad network, it covers more than 80 percent of the mobile Internet, according to the company, citing data from Nielsen. That means its ads reach around 73.5 million mobile customers.

Looking at device makers rather than OS vendors, Apple is still in the lead, commanding a 25 percent share of all ad impressions on the network. The iPhone kept its top spot with a 16 percent slice of all impressions, while the iPod Touch was in third place with 7.5 percent. The iPad also has shown solid growth, with ad requests on the tablet jumping 112 percent over September.

For October, Millennial Media expanded the coverage in its report to include all connected devices on its network, not just phones, allowing it to look at the iPod Touch, the iPad, and other gadgets, such as the Sony PSP portable game console.

Samsung's share of ad impressions grew 3 percentage points to capture 17 percent, putting it in second place behind Apple. Samsung's rise was due largely to demand for three phones: the Android-based Vibrant Galaxy S, the lower-end Samsung Smooth, and the Samsung SCH-R451.

Motorola took third place among device makers with a 15 percent impression share and two of its new Android phones--the Droid 2 and Droid X--in the ranks of the top 30 devices. Research In Motion also showed some nice gains with six of its BlackBerry devices in the top 30 list and the BlackBerry Curve in second place with a 7.9 percent share of all ad impressions.