not working with the_excerpt, function?

It’s working ok in single.php but not in index.php with the_excerpt() even though excerpt is enabled. I tested on default twenty twelve theme with no plugins installed (3.5.1 and 3.5.2).

Any way to include it as function? addthis_display_social_widget() – says missing parameter ?
Adding the filter in functions.php for excerpt is not working.

Thanks.

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