If you have bought it (and considering that a few million people did, you probably have), then there's good news for you, too. The game just got a free update that added Game Center integration to the mix, and Halfbrick included network multiplayer, which is something we haven't seen a lot of yet. Load up the update, open up the game, and you'll be able to jump in and play with anyone, or send one of your Game Center friends an invite directly.
And Halfbrick says more updates are on the way, including a "Secret Banana" mode and more Game Center functionality. So, if you haven't given Fruit Ninja a shot yet, now's a great time to do it.