Flurry monitors usage of more than 10,000 developers’ applications on iPhone and Android platforms. In total, Flurry tracks applications on approximately four out of every five iPhone and Android handsets in the market, generating over 25 million end user sessions per day. To estimate first week sales totals for the Nexus One, myTouch 3G, Droid and iPhone 3GS, Flurry detected new handsets within its system, and then made adjustments to account for varying levels of Flurry application penetration by handset. Flurry additionally crosschecked its estimates against Apple actual sales, released for iPhone 3GS, which totaled more than one million units over the three days, June 19 – 21, 2009. Flurry first week sales estimates can be found in the table below.

Early sales data for Google’s Nexus One are in and they seem to belie the “superphone” superlative the company has attached to the device. According to market analytics firm Flurry, Google sold an estimated 20,000 units in its first week.

An unremarkable showing, to say the least–especially for a device launched amid so much buzz. Flurry estimates that Nexus One was outsold by Motorola’s (MOT) Droid by more than 12 times and by Apple’s (AAPL) iPhone 3GS 80 times.

Now, Flurry’s Nexus One sales estimate is obviously the roughest of guesses–the company gets its numbers by monitoring application usage on the iPhone and Android platforms, not from any hard sales data. That said, it’s likely at least directionally correct and suggests that Google (GOOG) may have a tough time moving the five to six million Nexus One handsets analysts have been calling for it to sell by the end of 2010–especially if customer support issues associated with the superphone continue.