WebWare has a nice write-up on the wonderfully designed and amazingly functional iPhone-optimized version of Facebook, noting that the service takes advantage of Web-based dialing functionality to serve as a phonebook and contact manager of sorts for the iPhone.
"Like the full version of Facebook, the iPhone iteration centers around your profile, friends, messages, and the home tab, which contains the Facebook news feed along with any upcoming events and notifications. You can browse profiles and check out your friend's links (which open up in new Safari windows). The real killer app though is the call option, which will pop up assuming your friend has made their phone number available. This works with e-mail addresses too, but the phone numbers basically turns Facebook into one big mobile phonebook for your friends."
This is what a real iPhone Web app should look like.