The connection between Postcrossing and Facebook must be disabled on your Facebook privacy settings. The location of this settings might depend on your Facebook setup, but generally these are the steps to reach it:

• Login in Facebook
• On the 'Account' menu, go to 'Application settings'
• In the 'Show' dropdown, select 'Authorized'
• Scroll down to the 'External websites'
• In the Postcrossing item, click the 'X' to cancel the integration