Have a table in your own data model that uses a primary key UserName. This is the value that will determine which wall you show to the user.
When a user registers, after you create them using the ASP.NET Membership Provider, add a new row to the Users table giving the same UserName as the UserName in the membership provider (for simplicity, we use email address for username -- easier to guarantee uniqueness). You can execute queries for their wall data by using the User property of the Controller class.
- User registers, you create a user account for them (Membership.CreateUser)
- At same time as Membership.CreateUser, insert UserName into a row on your data model
- User signs in
- User requests page
- Page uses the Controller.User property to query your app database for the correct wall content to display.