The current Facebook mobile site is almost 1:1 with their app (on Android, anyway). I wouldn't surprised to know the app is one giant WebView with a few system hooks sprinkled in.

I don't even have it installed on my iPhone. I use the mobile site.

I never understood why websites created apps of their websites. The browser is ubiquitous. Use it. I don't want to install seven different apps for my seven favourite sites. I don't see why this is acceptable on a mobile platform any more than it would be on a desktop platform.