Scrabble requires a FaceBook account to play online, and you can only play against other FaceBook users. On the other hand any FaceBook user can play against you. No iPad required.
Joining open games from the iPad is difficult. Most games are filled and closed before you can finish joining them. Games you start yourself work fine. Games that you are invited to work fine.
If you want an almost identical game that does not need FaceBook, try Words With Friends. I have not used it myself, but it gets very goo reviews.
A faster word game with a stronger strategy component is Letterpress. I do have and use this one and love it. The only problem is the people who quit making moves when they realize they'er doomed.