I just installed the Ultimate Facebook plugin on one of my sites, and thought I would share this for those who may need to create an FB app for whichever WPMU Dev plugins need it (as per the instructions - All WPMU Dev plugins that need an FB app share the same one, so you only need to create one...)
This may be pointed out somewhere buried in a thread - or perhaps it would be good to update the instructions to save others from confusion...
#1) Be sure that the case of the what you enter in "App Domains" and "Website with Facebook Login" fields is the same. IE - if you enter MyDomain.com in the first, enter http://MyDomain.com in the second.
For many of you, this probably is a non issue, but I tend to Initial Cap my domain names so as to make it more legible for humans. Consistency is key or you will get an error that is misleading.
#2) The guide fails to mention the fact that after you create your App, it will be in Sandbox Mode and will not correctly connect with the plugin back in your WordPress WPMU Dev plugin page.
Create your app as per instructed and then in the App setting, Basic Mode area (upper part of screen), under Hosting URL, make sure that Sandbox mode is "Disabled".
Not too tough, but if you have never done this before, you can be confused.