Shortly after tying with Apple's iOS for first place among smartphone operating systems hitting Millennial Media's mobile advertising network, Android is now firmly in first place.

In its report on December traffic, Millennial says that Android phones in the U.S. accounted for 46 percent of traffic on the ad network, more than Apple's 32 percent. In October, Android tied with iOS, with each OS generating 37 percent of the traffic.

Since Android runs on many different smartphones, Apple is still the number-one hardware maker in the network, with nearly 21 percent market share, Millennial said. Samsung, which makes a number of Android phones, comes in second with 17 percent and HTC is in third place with 15 percent.

Research In Motion had five devices among the top 30 with a combined share of 11 percent in December, Millennial found. The BlackBerry Curve was the second-most-popular smartphone model during the month, with a 6.5 percent share.

Devices running the Windows OS made up just 1 percent of traffic; handset makers started selling phones with the Windows Phone 7 OS in November.

Android is still the OS growing at the fastest clip, indicating that it should hold onto its lead for some time. Requests to Millennial's network from Android phones grew 141 percent from the third quarter to the fourth quarter 2010, Millennial said. RIM requests were up 60 percent during that same period.

Apple's growth was considerably slower, at 12 percent from the third quarter to the fourth, but that growth rate could soon spike once the iPhone becomes available on Verizon's network in February.

Millennial says that it offers the largest data set of any independent mobile advertising network in the U.S. AdMob used to release similar regular reports of mobile traffic hitting its network but stopped shortly after Google acquired the company.