A week ago, I wrote that only three of Apple’s top twenty grossing iPhone Apps feature in-app purchases. I wondered whether developers have determined that direct sales were more dependable and profitable than relying on user-engagement to derive revenue.

Proof it can be done the other way: Tapulous’ hit iPhone application, Tap Tap Revenge, is a free application… and it has now cracked the top twenty-five grossing apps – purely through the in-app downloads / purchases.

Tap Tap Revenge went free just before Christmas and had over 2,000,000 downloads in the subsequent week:

Tapulous, developers of the popular Tap Tap Revenge series, check in with good news. They’ve pulled down over 2 million installs of Tap Tap Revenge 3 since going free last Wednesday, 700,000 of which came on Christmas day. Between Tap Tap Revenge 1/2/3 and the Metallica/Lady GaGa editions, Tapulous now has 5 applications in the Top 100 grossing apps. (More on TechCrunch from Daniel Brusolovsky)

Tap Tap Revenge’s top in-app purchases are all ‘packs’: nine of the top ten are either two or six-track downloads. Two-packs are $0.99 and six-packs are $2.99. Interestingly, the only individual track is the Avatar theme song (Avatar also has the #22 top-grossing app at $6.99). The paid applications are in addition to the weekly free apps released each Thursday:

Tap Tap Revenge comes with more than 40 free songs! Best of all, we offer Tap Tap Thursdays: every Thursday we feature a FREE hot new or exclusive track from top artists. So far we’ve featured songs by Katy Perry, Anberlin, Everlast, Michael Franti, Lady Antebellum, Lee Perry, 3OH!3 and Bitter:Sweet, to name just a few. Check out the full list here.

Not that it is directly related to the their revenue success, but Tapulous also does a great job communicating with their users through their outstanding blog. It is regularly updated with status updates, contests, new music, etc. But despite having over 4,000 Facebook Fans, their Fan Page is totally empty:

Popularity: 2% [?]