I had the same problem. It has to do with caching on Facebook’s side of things. Take the page and run it through the Facebook Debugger here: http://developers.facebook.com/tools/debug

It fixes it right away.

Woops, figured my characters would be escaped automatically… Here it is again:

Consider all event slugs being >nation</events/>slug<

Or blog posts:
>nation</blog/>year</>month</>slug<

Or planks in a campaign platform:
>nation</platform/>slug<