#1. Zynga: 40,000,000+ monthly active Facebook users #2. RockYou: 40,000,000 monthly active Facebook users #3. LivingSocial: ~27,500,000 monthly active Facebook users #4. Slide: ~25,000,000 monthly active Facebook users The trends are as remarkable as the numbers (consider that 20,000,000 actives represents 10% of all Facebook!).
Zynga is growing steadily; RockYou has stabilized and Slide has dipped. Meanwhile, the LivingSocial growth rates are phenomenal. The below chart is over one month's time:
It will be interesting to see if LivingSocial supplants Zynga at the #1 spot and where / when the growth flattens.
The full charts are shown on InsideFacebook.com. I encourage you to take a look because it puts the velocity of InsideSocial's growth in perspective.