Advertising impressions on tablets are growing twice as fast as they are on smartphones, according to mobile advertising network InMobi.

Over the last six months InMobi has seen the number of tablet impressions grow by 88 percent on its network. Android devices now account for 28.9 percent of all tablet impressions the company serves, up from 18.2 percent in Q4 2011.

InMobi credits the growth in Android tablets almost entirely to the success of Amazon’s Kindle Fire, which alone accounts for 9.2 percent of impressions. The Kindle Fire is the second most popular tablet in the market, behind Apple’s iPad. The third most popular tablet is the Asus Eee Pad Transformer, which accounts for just 5.2 percent of InMobi’s tablet impressions.

Overall, InMobi’s numbers echo the results of another mobile advertising company’s latest study. Competitor Millennial Media’s latest Mobile Mix Report noted impressions on non-phone connected devices (i.e. tablets, e-readers and MP3 players) were up 33 percent quarter-over-quarter. In that category, Millennial reported the iPad was the most popular device, followed by the Samsung Galaxy Tab, with Amazon’s fast-growing Kindle Fire taking the No. 3 spot.

With two of the biggest independent mobile advertising networks both highlighting the growth of the Kindle Fire’s market share among Android tablets, it’s safe to assume if Amazon’s device can continue its momentum it will split the Android market, making Amazon App Store the best way to reach Android tablet owners.  With developers like Crowdstar and TinyCo reporting the Amazon App Store is capable of delivering higher average revenue per user that both Google Play and the iTunes App Store for iPad, it seems prudent for Android developers to begin exploring their options on Amazon if they wish to participate in the Android tablet market at all.