Mobile advertising impressions are increasing more quickly on tablets now than they are on smartphones according to Millennial Media’s quarterly Mobile Mix report.

During Q1 2012, impressions on non-phone connected devices — a classification that includes tablets, e-readers and MP3 players like the iPod Touch — grew by 33 percent quarter-over-quarter. Non-phone devices now account for 20 percent of all impressions Millennial serves, up from the 15 percent share the devices held in Q4 2011.

According to the mobile advertising company, tablets made up most of the growth in the non-phone category, with three tablets — the iPad, the Samsung Galaxy Tab and the Kindle Fire — ranking in top 20 mobile devices by impressions served. The result is a significant increase year-over-year, particularly for Android tablets. During Q1 2011, the only tablet to make the company’s top 20 device list was the iPad.

Millennial’s top five tablets by impression during Q1 were the iPad, the Samsung Galaxy Tab, the Kindle Fire, the Motorola Xoom and the Blackberry Playbook, an interesting result considering a recent ComScore study indicating the Kindle Fire accounts for more than half of all Android tablets in use. While the Samsung Galaxy Tab is still the most common Android tablet according to Millennial, the Kindle Fire could overtake it if the device maintains its growth rate. This would make the Amazon Appstore the main gateway to the Android tablet market.

Aside from tablets, the iPhone and iOS are still at the top of Millennial’s manufacturer and device list. Apple manufactured devices accounted for 28.32 percent of all impressions Millennial served, and the iPhone family was the most popular set of devices overall, accounting for 15.10 percent of all impressions. Apple’s closest competitor among devices during Q1 was RIM’s BlackBerry Curve, which captured 4.44 percent of all impressions served.

Overall, Android was still the most popular operating system, accounting for 49 percent of all impressions and accounting for 15 of the 20 most popular mobile phones. Apple’s iOS accounted for 33 percent of total impressions. RIM’S BlackBerry OS, Windows and Symbian continued to see their share of mobile ad impressions decline, dropping to 14, 3 and 1 percent respectively.