Mobile ad network Millennial Media, whose ads reach 63 million of a total of 77 million mobile web users in the U.S., or 81% of the U.S. mobile web; is releasing its monthly mobile mix today. In September, Android was again the second largest Smartphone OS on the network since July 2010, increasing 2% month-over-month to a 29% share of impressions. Apple’s iOS took the top spot, with 49% of impressions on Millennial’s network, up 1%