From a post on the Seesmic Blog:
Over the weekend, we released a feature in Ping.fm that will allow you to post an RSS feed to all of your social networks. All you simply need to do is choose the RSS selection under Services/Tools (right there in the Dashboard), and then enter in your RSS feed to the blog of your choice.
Powered by Superfeedr, once the post of the blog is published, Ping.fm within minutes will send the status update to all of your social networks.
This feature currently supports one RSS feed, and we’ll be working on the ability to add additional feeds, as well as other configurations, so keep an eye out. We’re pretty excited about this feature and looking forward to providing more for Ping.fm.