Extending membership model but userid return zero using LINQ

I want to implement a DAL to access and extend the ability of membership model.
I currently use userid of a membership as the FK for my objects which should belongs to a user/membership.
Since the userid is the PK for the membership, that's why I use it. If membership has a membershipId, I will use that instead.
Then I come across two questions,
1. a user should have a membership in each application, how come the membership table is using userid as PK?
2. I am using LINQ instead of the membership API to get my user information, but the return aspnet_Membership object's userid is zero GUID, why is that?
How can I get a aspnet_membership object with the correct userid?
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