MarieCT, thanks, I had the same issue. For further improvement use a try/catch block or just a class_exists to avoid things breaking in the theme files if this particular plugin is not around, something like
<?php
if (class_exists('MR_Social_Sharing_Toolkit')) {
$social_sharing_toolkit = new MR_Social_Sharing_Toolkit();
echo $social_sharing_toolkit->create_bookmarks();
}?>
