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Since Blurb implemented this "feature" I have stopped commenting on books as I do not want to join Facebook either, it’s terms and conditions and approach to use of my material are appalling. As an example their Ts & Cs state the default privacy is…

…..you specifically give us the following permission….. you grant us a non-exclusive, transferable, sub-licensable, royalty-free, worldwide license to use any IP content that you post on or in connection with Facebook….

I raised the change with Blurb Customer Support and got the following response which confirmed the restriction…..

Thank you for your email in regards to you wanting to leave a comment. I apologize but the only way to leave a comment is if you are a member of facebook.

…..Tony