To add a framework to a library, you need to do two things:
1. add the framework to the libs of the target that depends on it (libs += [ "Facebook.framework" ])
2. add the framework to the framework search path, by adding it to the cflags and ldflags of the target (common_flags = [ "-F", path_to_dir_with_framework ], cflags += common_flags, ld_flags += common_flags).
Usually this is done by defining a config object and then adding this to the target configs property. See for example how xctest_config is defined in src/build/config/ios/BUILD.gn.
-- Sylvain