Social Share buttons aren’t available in the Blog now, but we have a request for this on the Wishlist - Social share at bottom of blog post.
Basically, you can share links to specific blog posts. You just click on the needed post, the post popup appears, and you get a link to this post with the URL slug added to the widget:
However, due to the peculiarities of the Google Sites platform, this feature won’t work there. I am really sorry 
Thus, the only option for now is to use the link to the whole page where the widget is installed, or share just the widget using the Share Link option:
For the same reason (Google Sites iframes) the Social Share Button widget doesn’t work correctly.
I am not sure if it can be adjusted, but if you could share a link to the page where this widget is installed, I’d be happy to double-check it with the devs.