Wirelessly posted (Mozilla/5.0 (iPhone; U; CPU iPhone OS 3_0 like Mac OS X; en-us) AppleWebKit/528.18 (KHTML, like Gecko) Version/3.1.1 Mobile/7A259g Safari/525.20)
If it's only one third party application, then I'd choose BeeJive (or meebo, since it already technically has push and I'm on 3.0 which crashes BeeJive every two seconds).
Sudoku, eBay, Amazon, facebook, etc. all can be accessed from Safari, abeit very badly, but better than the instant messengers.
Why would you pick that as your only app when Safari can take you to the Facebook iPhone site? Assuming we are allowed to still have all the standard iPhone tools.