Facebook runs a system called the Open Graph. When you post one of your podcast URLs to Facebook, their servers scrape the Open Graph. metadata we provide, to display a card on your timeline.
Once Facebook has grabbed this information and you post the same content again, Facebook caches the description, even if you change it in Podiant. Here's how to clear the data in Facebook.
Use the Open Graph Debugger tool, and enter in the URL you need to update (for example, your podcast's homepage). Click "Debug" and you'll see a snapshot of all the information Facebook has scraped from your page. Click the "Scrape again" button, and Facebook will return to your URL and grab all the updated metadata. From now on, any time you share that URL to Facebook, users will see the updated info. If you change your social settings (either for your podcast in general, or for individual episodes or blog posts), you'll need to go through this process (not for every page, but just the ones you need to change).