Apple (AAPL) is still the No. 1 device manufacturer in Millennial Media‘s ad network with a 25% share, well ahead of Samsung (17%), Motorola (15%) and Research in Motion (13.5%).

But its iOS operating system no longer dominates the smartphone ad space. Google’s (GOOG) Android, which had a 17% share to Apple’s 56% in Millennial’s June survey, finally caught up in the October report issued Thursday.

The two major mobile OS competitors are now even with 37% each, trailed by RIM (RIMM) with 20%. Nokia’s (NOK) Symbian, which dominates the world smartphone market, is in a three-way 2% tie in the U.S. with Microsoft (MSFT) and Other.

Since Google acquired AdMob and Apple swallowed up Quattro Wireless, Millennial is the largest independent mobile ad network standing. It claims to reach 54.3 million users, or 81% of the U.S. mobile market.

We’ve collected five months worth of Millennial Media pie charts below the fold to show how the smartphone ad market has shifted since early summer. If current trends continue, Android should have the largest slice in Millennial’s next report.

[Follow Philip Elmer-DeWitt on Twitter @philiped]