Fatal error message on blog after installing facebook app

i have tried installing the facebook app but now im gettign this error message whenever i click on a blog post:

Fatal error: Call to undefined function wp_trim_words() in /var/www/vhosts/main-source.co.uk/httpdocs/blog/wp-content/plugins/facebook/fb-open-graph.php on line 45

any help would be appreciated

thanks

http://wordpress.org/extend/plugins/facebook/