Author hide a 1×1 pixel as backlink to his site…

I’m pretty sure this kind of code is prohibited to get integrated in the WP plugin directory anyway.

Correct. Flagged for investigation.

Okay, so this is acting as service (i.e. they’re running the share stuff through their servers, just like Jetpack does and Add-to-any and share-this). So while it’s odd, it’s okay with the JS.

The 1×1 pixel is less so.

This plugin is acting as a service, so it behaves exactly like other sharing plugins (Add This, Share This, Add to Any). This is explained in the plugin FAQ, but let me be even more specific.

dot.png is just a special version of “share image”, used to replace the original share image, when user chooses to hide the share image in plugin settings (by default english share image is shown). Because the option to hide the image was added later, on user request, the script still needs img element there to operate properly. So it gets replaced by minimal img element possible – dot.png. All images have links that point to external sharing services (such as Facebook, Twitter). Links redirect throught add.php (again, the same behaviour as with other major sharing services) as this has one major advantage for WP users: they do not have to update the plugin every time a sharing services change their sharing urls. The changes are only made on the server side in add.php. Since many webmasters do not update their plugins after the website is finished, this is a great benefit to them, because the plugin keeps working without updating, even when social networks change their urls/apis.