Arguments to xcodebuild are best used for values that change every time they're invoked, like a destination directory that includes a build number.
For data as foundational as a Facebook app ID, I'd recommend using your build configurations, e.g. "Debug" and "Release". Create a user-defined build setting (in Xcode 5.0.2, Editor -> Add Build Setting -> Add User-Defined Setting) named something like "FACEBOOK_ID", and in your Info.plist, set your desired key's value to ${FACEBOOK_ID}. In your target's build settings, define FACEBOOK_ID differently for your Debug and Release configurations. Your code then now has to pull the current value out of Info.plist at runtime.
If you really want to override build settings as arguments to xcodebuild, you can do so, just by adding "FACEBOOK_ID=12341234" to the end of the command, but this only works if you've done the work I just described to make FACEBOOK_ID into a configuration-specific build setting. I can't think of any sound reasons to keep this kind of app data outside of a build configuration.
If build configurations are new to you, I'd suggest starting with WWDC 2012 session 408, "Working with Schemes and Projects in Xcode".