Apple devices account for 4 out of 10 online ad impressions by Philip Elmer-DeWitt @FortuneMagazine September 6, 2013, 12:53 PM EDT E-mail Tweet Facebook Google Plus Linkedin Share icons Click to enlarge. FORTUNE — Ad impressions as reported by Millennial Media MM , the largest independent online ad network, offer an imperfect measure of market share. But they do indicate the way things are trending. The chart at right, for example, taken from the MobileMix report that Millennial issued Friday morning, shows that Google’s GOOG Android now accounts for more than half (51%) of the ad impressions in the network, but that the share represented by Apple’s AAPL iOS grew faster year over year (to 42% from 34%). The continued strength of Apple’s hold on the tablet market is indicated by the chart below, which shows Apple’s iPad with a 55% share and the rest — Samsung, Google, Amazon AMZN and Others — fighting for the remaining 44%. Click to enlarge. Bottom line: Apple remained the top manufacturer in Millennial’s ad network, with its share of total impressions growing to 39.2% in Q2 2013. Samsung was number two at 26%. Apple also had three of the top four devices on the platform — the iPhone (#1), iPad (#3) and iPod touch (#4) — while Samsung had seven of the top 20, starting with its Galaxy S series of smartphones (#2). But you can’t draw too many conclusions from ad impressions because for a variety of reasons the owners of some devices spend more time online than others. In Q2, for example, 79% of the smartphones sold worldwide were Androids, according to IDC, and only 13% ran iOS, yet iOS’s share of Web traffic in August was nearly 55%. Click to enlarge. That discrepancy is underscored by the spreadsheet at right, published Friday morning by Piper Jaffray’s Gene Munster. It shows Apple’s share of Web usage in the U.S. growing to more than 65% while Android lingers in the 30% range. Munster offers three reasons for this: (I quote) First, we note that with over 50% share at both AT&T and Verizon in the June quarter (third full quarter of iPhone 5), we believe iOS is the most popular smartphone platform in the U.S.. Second, we believe that iOS users are likely more engaged with their phones on a daily basis than Android users on average. Third, we believe that iOS’ contribution from tablets, which are likely to generate more traffic than a smartphone, is currently greater than Android’s. LINK: Millennial Media’s MobileMix Q2 2013.