Facebook Plugin and Domain Mapping


Im justing starting at make new project, where I will be have 5 different domain (Blog, Market, Gallery....), they will all be based on Wordpress so I thought I would use Wordpress Multisite.

The reason for this is they are all going to be linked and I want to share the same user database. However I have just tried to sent it up now, just with two domain, using the plugin Domain Mapping for number two, however when I register a user through the facebook plugin that user is only logged in on the one domain, and not on the next.

A normal wordpress user i logged in on all, I guess this has something todo with either cookies or facebook not allowing it ?

Anyone who have expirence this, or even know a fix for this ?

I was just thing when I was writing this, would I better let of by creating 5 different wordpress site and then use the user sync, and would it even work ?