I have my blog set up to use a page as the homepage and has an additional page as the blog index.
Checking the show on homepage option in the advanced settings shows the AddThis buttons on the homepage even if the show on pages option is unchecked. Without checking the homepage option there is no way to show on the blog index.
I've fixed this by amending addthis_social_widget.php to check if the homepage is a page before deciding to display (and using the show on pages setting). It would be great if this could be fixed in a future version, so that I can keep the plugin up-to-date.