How can I add a Socit! button to my wordpress.com blog?

It looks like you've run into the reason why so many of us use Wordpress.org - you can't install plugins in a Wordpress.com blog, and you can't paste javascript into your template or a text widget. In other words, you are incredibly limited by how much you can customize your blog on Wordpress.com.

The reason is explained by Wordpress.com themselves:

Users are not allowed to post JavaScript on WordPress.com blogs. Javascript can be used for malicious purposes. Your code and intentions may be perfectly harmless, but it does not mean all javascript will be okay. The security of all the blogs is a top priority and until we can guarantee scripting languages will not be harmful they will not be permitted.(http://support.wordpress.com/code/)

Blogger isn't quite as strict as WP.com. Pasting the Javascript into a Blogger template isn't ideal, but it is possible, as Tom shows here: http://www.sixmats.com/2008/07/what-h...

If you're blogging simply for the purposes of writing down your thoughts and sharing them with family and friends, Wordpress.com is sufficient, but if you're intending to attract an audience and promote your articles on social networks, whether it's JapanSoc, Twitter or Facebook, you might want to consider switching to WP.org before you get too far into your blogging. You can be up and running for less than $20 and then about $6 a month after that, and you know what? It's not nearly as difficult to run a WP.org blog as you might think! :)