Based off a population list on Wikipedia. Zynga's follow-up to their casual mega hit FarmVille has broken barriers according to Mashable.
AppData, a site that specializes in Facebook (Facebook) app metrics, has CityVille at 99.9 million monthly active users, but since (by AppData's count) CityVille has been adding nearly 400,000 new monthly active users every day, the 100 million MAU milestone will surely be reached in the next update.
I confess I have never played CityVille or FarmVille. I avoided them like some sort of plague but now...now I am curious. What waits for me on the other side? Sword in hand I do declare it adventure time.
Feel free to comment if you can explain what would take 100 Million people's attention by storm.