According to the latest data from Millennial Media (PDF Link), Apple’s share of overall U.S. mobile device ad impressions is on the decline. In May, Apple devices accounted for around 25% of all impressions, down sharply from 35% the month before. The report gives no indication as to whether Apple’s changes to the iOS developer agreement, which banned the sharing of user and device data with third party ad networks and analytics tools, had any affect on the results. Also of interest in the report is a new section on mobile developer trends. The report claims that 90% of all U.S. developers are single-platform, with 56% of those focused on iOS development, followed by 29% that are focused on Andoid. Millennial Media’s numbers are based on impressions served on its network of mobile sites and apps, which reaches 82% of the U.S. mobile audience, according to the company.