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%.

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%.

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)