We should at least have a property in Facebook.FacebookApplication that we can set in web.config. This is important. When a user signs into a tab page application with OAuth the redirectUrl parameter needs to be populated, typically the tab page URL that originated the OAuth sequence. We are storing this in appSettings, but with other Facebook settings in the <facebook> node, so you have a disparity of where to store Facebook details. We have a ton of other settings to worry about; Facebook settings should be kept together especially when the settings requirements are consistent across multiple applications.

Seems the tab page can be constructed as something like:

// implement ResolveRedirectTarget() manually as invoke via HTTP to get redirect target
var pageBaseUrl = ResolveRedirectTarget(scheme + "://www.facebook.com/" + fbrequest.page.id);

// combine base page URL with application ID as tab querystring param
var tabPageUrl = pageBaseUrl + "?sk=app_" + Facebook.FacebookApplication.Current.AppId;