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%.
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.
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)
Calls Apple's iPad mini the "must have gift" of the year
FORTUNE -- "The iPad mini is a game changer," writes Millennial Media's Matt Gillis in a blog post Monday.
"Every holiday season there seems to be one particular item that stands out as the 'must have gift' of the year," he writes, "and in 2012, early figures lead us to believe that this could be the iPad Mini. Truth be told, MOREPhilip Elmer-DeWitt - Dec 10, 2012 12:34 PM ET
The Samsung Moment? The Motorola Cliq? The HTC Magic? The BlackBerry Tour?
Maybe I don't spend enough time shopping for Google (GOOG) Android phones, but I always get a kick out of the Top 20 Mobile Phone lists -- measured by ad impressions -- that Millennial Media posts each year.
In the 2011 report issued Thursday there weren't many surprises at the top of the list.
Apple's (AAPL) iPhone, the No. 1 best MOREPhilip Elmer-DeWitt - Feb 24, 2012 10:24 AM ET
iOS edged back up against Android in November, gaining 8% in Millennial's ad network
There's a small but measurable iPhone 4S effect in November's Millennial Media's Media Mix, which reports the presence of various devices in the mobile advertising network it runs.
Apple (AAPL) was the No. 1 manufacturer in Millennial's advertising platform, up 8% since October to gain a 26% share in the list of the top 15 manufacturers.
Meanwhile, in Millennial's monthly pie MOREPhilip Elmer-DeWitt - Dec 20, 2011 6:03 AM ET
Among the top 20 smartphones in Millennial's March '09 report, only two are still standing
Here's a pair of interesting lists from Friday's Mobile Mix -- the 50th "intelligence report" Millennial Media has issued since the mobile advertising network opened for business five years ago.
If you click to enlarge the chart at right, you'll see that in March 2009, Apple's (AAPL) iPhone was No. 1 among the top 20 smartphones in MOREPhilip Elmer-DeWitt - Aug 12, 2011 6:11 AM ET
But in the smartphone race, Android's lead grew slightly, according to Millennial Media
There's good news and bad for Apple (AAPL) in Millennial Media's May report, issued Wednesday morning.
The iPhone is still the top phone in Millennial's ad network, for the 20th month in a row
Apple is still the leading mobile manufacturer, with a 30% share
When breaking down ad revenue driven by apps, iOS devices were still narrowly ahead of Google MORE
iPhone ad impressions grew 17% from February thanks to the Verizon launch
Google's (GOOG) Android still represents the biggest slice of the smartphone pie in the monthly report issued Thursday by Millennial Media, the largest independent mobile ad network.
But for the first time since 2010, when Android began dominating Millennial's Mobile Mix reports, Apple's (AAPL) share of smartphone ad impressions grew a bit in March (from 27% to 31%) and Android's MOREPhilip Elmer-DeWitt - Apr 14, 2011 7:08 AM ET
Breaking the tie from last quarter, Android is now way out in front of iOS and the other smartphone vendors in terms of impressions measured by one agency.
Google's (GOOG) Android saw some strong numbers today as Millennial Media reported its advertising share makeup. As far as apps are concerned, Android carried 55% of impressions for the month. Apple's iOS was second with almost 40%.
Android's impression share was less dominant but still MORESeth Weintraub - Jan 13, 2011 1:33 PM ET
For the second month in a row, two smartphone operating systems are in a dead heat
The extraordinary growth of Google's (GOOG) Android phones can be traced in six months of smartphone advertising data from Millennial Media, the largest independent mobile ad network.
In a series of pie charts, they show Android overtaking Research in Motion's (RIMM) BlackBerry between July and August in ad impressions, gaining rapidly on Apple's (AAPL) iPhone between MOREPhilip Elmer-DeWitt - Dec 14, 2010 6:10 AM ET
In Millennial Media's latest report, Google's OS has finally caught up to Apple
Apple (AAPL) is still the No. 1 device manufacturer in Millennial Media's ad network with a 25% share, well ahead of Samsung (17%), Motorola (15%) and Research in Motion (13.5%).
But its iOS operating system no longer dominates the smartphone ad space. Google's (GOOG) Android, which had a 17% share to Apple's 56% in Millennial's June survey, finally MOREPhilip Elmer-DeWitt - Nov 18, 2010 7:05 AM ET
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