I joined Facebook two weeks ago when I discovered that it wasn't just college kids anymore.
I got hooked into Scrabulous almost as soon as I got on Facebook, thanks to a friend who convinced me to play a game with her.
When Scrabulous got shutdown by Hasbro for infringing on the Scrabble trademark, I migrated almost immediately to the official game. Since I've not been playing long enough to even finish a game, I have no great loyalty to Scrabulous.
Yet ... Scrabble's user interface is painfully amateurish. Whoever's been designing the application has been designing it like a desktop application. It does not take advantage of all the social networking that Facebook provides. It also is graphics-heavy and takes a long time to load. Scrabble does not work properly on Linux. Which may be for the better since it cuts down on temptation to sneak in a play at work.
Still ... my first two plays with Scrabble were: pardine/tinea and fomented. Two bingos (using all 7 tiles), for a grand total of 143 points. I think I like Scrabble.