We're looking in to Facebook integration as well and I've been reading about the subject intensely at http://developers.facebook.com and the Javascript SDK in particular (there is one or more .net and c# sdks floating around but I'm finding the js version is easier to implement) in order to create our own controls and login/logout methods that will also utilize the Kentico API to help manage this. I'm guessing that, essentially, you let Facebook handle the login/logout (it maintains session status for you to query for state) (and FB authorization) then use Kentico for more CMS specific personalization. Plus, it can enable a much deeper marketing reach through that very popular channel.

This effort will require you to create a Facebook "application", which is your website, and it then assigns you an "App ID" and "App Secret ID" which, in pair, you can then use to access the current user public data (such as an email) for reading. Before you can do that, you'll have ask the user for authorization for specific permissions such as accessing the basic information, posting to their wall and other functions.

There's much more at the link above but it's going to require some custom work on your part. We're also still on 5.5 as well.

Is ver. 6 planning on using the new oAuth 2.0?