When you created your forums did you include your FB App ID and FB
App Secret number in your set-up? It needs to be in your Bin/Config/Site.Config file under
<facebook apiKey="your numer here" secretKey="your secretKey here" />
<twitter apiKey="Your Twitter Key here" />
If you are setting up your forum using the Microsoft Web App Gallery, it should give you a place to enter that information on set up, but as shown above you can always enter it manually into your Site.Config file.
Hope this helps. :-)