I'd like to know more about this feature myself. Simply put, in All Settings->System->Apps for Websites, it has a section:
I would think that it would be rather convenient to have a link that attempts to open a website to actually open in the app for that site, such as Facebook, Twitter, Windows Central, etc. However, none of these apps, nor anything Microsoft has released, seems to register with this. You are unable to "assign an app", as it appears that you have to install an app capable of it.
I would love to know more about the plans for this feature, if anyone has info. Perhaps Daniel Rubino or some other person with an inside track can get some info for us.