Never heard of LOLapps? Don't worry - most of their 44,000,000 active monthly users probably haven't either.
LOLapps enables users to easily create custom Facebook Apps using their tools. The basic application templates are currently quizzes and gifts (with more robust gaming templates on the way). So while you have likely not heard of LOLapps (as you have heard of Slide or RockYou) - you have likely encountered a gift or taken a quiz that was built using LOLapps. In fact, they now reach 22% of Facebook users (44m monthly).
Yesterday I wrote about Facebook's Toop Developers by monthly reach. LOLapps doesn't appear in those reports because they do not show up as the developer of each application. Their 44,000,000 monthly uniques though would put them on par with Zynga at the #1 spot. As an example, take a look of one of their top Apps: "What Sex in the City Character are You?"
Nearly 200,000 users - which is a big number. But to reach 44m, you begin to realize how many long-tail quizzes and gifts have been created!