Slashkey’s Farm Town has been the runaway hit on Facebook, the current number one application and game on the social networking site. The torrid growth started shortly after its February 2009 launch, reaching over 4.5 million daily active users as of today, which I believe is a record number for any game application on Facebook.

While Slashkey’s rise to the top has been impressive, we’re also seeing unprecedented growth from competitor Zynga’s own Farm Simulation game, Farmville, which has already topped 3 million daily active users since it’s June 23rd launch, landing it in the #5 spot according to Developer Analytics.

While Zynga was able to cross-sell from other games like Mafia Wars (Slashkey doesn’t have any other Facebook applications), they also seem to be better at engaging users, seeing more Facebook status updates from the Farmville users than Slaskey is getting from Farm Town users.

To try to quantify the level of engagement by the users of each application, I used Facebook Lexicon data from June 29th (there is a lag so no more recent data is available) and the Facebook Lexicon Activity Pixel Index ™ (or FLAPI) that measures the number of pixels from the x-axis and to give you a number you can use to benchmark competing brands when using Facebook Lexicon graphs (see other posts using FLAPI).

So while not as impressive as Mafia Wars sporting over 100 on FLAPI per Million DAU last month, Zynga’s Farmville appeared to be engaging it’s users 32% better (32.4 vs. 24.5) than Slashdot’s Farm Town.

Finally, the more interesting take-away may be that instead of cannibalizing the existing Farm Sim giant, Farm Town, Zynga’s new Farmville game has actually expanded the genre, showing that two somewhat similar games can co-exist: in the last two weeks the two games have increased the daily active users by nearly 2.5 million per day or nearly 50%.

Disclosure: I have done consulting work for Zynga but have not been compensated or had any input from them to write this post