On Xbox 360, when you try to start facebook application, if you get following error 800700e8 you need to do following steps to start running Facebook application.
You will be requiring Xbox 360 gamertag which has country set to United States. If you dont have, you can create one gamertag and set its country to US. Login with US gamertag id on Xbox 360 and download Facebook Application.
Once download is done, you can sign out and you can start using your original gamertag and run the Facebook application, and error will not come now.