getAtPluginPromoText String concatenation

String concatenation $string . __('plugins for ');

Should be:

function getAtPluginPromoText(){
    ...
        }  else {
            $string .= __('plugins for ');

            ...

        return '<p class="addthis_more_promo">' .$string . '</p>';
    }
}

https://wordpress.org/plugins/addthis/