Google Inc. is launching a salvo against Facebook Inc., saying it will no longer allow the social network to grab information about Google users' social and professional contacts in Gmail, Google's email service.

Google has always allowed Google users to transfer data, including their contacts, to other websites. Until now, new Facebook users could find out whether their contacts on Gmail also had Facebook accounts, simply by typing in their Gmail user name and password as part of the Facebook signup process.

That...