Omgpop (formerly i'minlikewithyou or iilwy) is a casual online multiplayer gaming website built entirely in Flash. Omgpop states on its homepage that it is "a place to hang out, play free online multiplayer games, and make friends."
Omgpop's most popular game to date is their Pictionary-like game titled, "Draw Something." The game has done so well, that on March 16th, 2012 it was reported that it was the most played game on Facebook, registering 10.8 million daily active users, compared to Zynga's Words With Friends, garnering 8.6 million daily active users respectively.
Source: Wikipedia, "Omgpop", available under the CC-BY-SA License.