ComScore issued its May report on mobile subscriber market share in the U.S. last week, and BGR noted that Google’s Android OS and Apple’s iOS grew more popular in the month, mostly at RIM’s expense. Perhaps equally as troubling as RIM’s continued slide, however, is Microsoft’s position with Windows Phone. As noted by winrumors, Microsoft’s share of the U.S. smartphone market has dropped a whopping 35% since it released its next-generation smartphone platform in November of last year. At that time, Microsoft’s Windows Mobile accounted for an estimated 9% of smartphones sold in the U.S. In May, Microsoft’s combined share was just 5.8%. We’re big fans of Windows Phone here at BGR, but the lack of carrier attention has left the OS as something of an afterthought thus far. With Windows Phone 7.5 “Mango” devices due later this year and Nokia set to launch its first Windows Phone before 2012, Microsoft is well-positioned globally. Here in the U.S. however, the Redmond-based tech giant needs far more carrier support if it hopes to make a dent in an already crowded smartphone market. ComScore’s May smartphone platform figures follow below.