It seems that the compressed version of Prototype I included in the Share This 1.3 download isn’t compatible with IE7. I’ve repackaged with the original (larger) version and will attempt to get a more compatible compressed version for a future release.
Note the updated FAQ section of the README in this release as well.
The download is available on my WordPress Plugins page.
If you have any trouble with this, please open a thread in the WP Support Forums and send me the link.