BO > Modules > Uninstall then delete. BO > Advance Parameters > Performance - Clear cache(located at top right). Download prestashop 1.6.06, unzip then to folder prestashop > modules and zip folder blockfacebook. FTP blockfacebook.zip into "Your prestashop diectory/modules/" and unzip it. BO > Modules > Install ! You might have to re-position the module in BO > Modules > Position.
finally it works for me now