Mobile advertising firm Millenial Media on Wednesday published its Mobile Mix report for the month of May, highlighting several interesting developments in the mobile market. Using ad impressions served to each operating system as a barometer for share, Millennial can give us a good assessment of the current smartphone and connected device markets. Android was flat compared to April maintaining its 53% share of Millennial impressions, and Apple’s iOS lost one percentage point in May as it slid to 27%. The iPhone still held an overwhelming lead as the most popular single device for the 20th consecutive month, however. Apple is also Millennial’s top-ranked manufacturer, with 30.84% of all ad impressions served to iOS devices. Samsung found itself in the No. 2 spot in May with 13.59%, followed closely by RIM at 13.30%. HTC and Motorola rounded out the top-5 with 10.98% and 9.47%, respectively. While Microsoft’s Windows Phone platform is still barely a blip on the radar, impressions to the OS did nearly double month-over-month, growing 92% compared to April. Apple’s iPad was the most popular “connected device” in May, growing 29% month-over-month. Hit the break for some more key data from May’s Mobile Mix report.