Superfeedr, which powered the RSS-feed addition, is also behind the Google Reader integration.
From the Seesmic Blog:
Along with Ping.fm recently adding our Feeds feature, we’ve just released the ability of adding Google Reader as another service. Now users are able to choose to share items on Google Reader and automatically update to dozens of social networks!
With the help from the team at Superfeedr, which supports our feeds feature of adding your RSS, Atom, or other variants of feeds, you can now add your Google Reader feed and have your “shared items” post instantly to your networks.
In Ping.fm, simply enter your Google Reader user name. Then, when you see a post or article that you’d like to share in Google Reader, simply click the “Share” button and Ping.fm will automatically send to all your assigned networks.