The problem:
Compiling Facebook SDK with Xcode 6 is not yet an option. swiftsh build_all.sh gives the following error:

AttributeError: 'NoneType' object has no attribute 'group'
Command /Users/../Vendor/clang-as-ios-dylib/links/cc-iphonesimulator-latest-targeting-5.0 failed with exit code 1
\*\* BUILD FAILED \*\*

The solution:
The SDK can easily be build with Xcode 5. We renamed Xcode.app to Xcode_5.app prior to installing Xcode 6.
And then used code-select to force sh build_all.sh to use Xcode 5.

sudo xcode-select -switch /Applications/Xcode\_5.app/Contents/Developer