DataSource Controls :: Fetching User Properties With Querystring With Function?

I found this function on [URL] It works great and I can get the properties and display them in separate binds. This is how I call the function, as shown In the link above

SELECT userid, username,
dbo.aspnet_Profile_GetProfile(userid, 'FirstName', DEFAULT) AS FirstName,
dbo.aspnet_Profile_GetProfile(userid, 'LastName', DEFAULT) AS LastName
FROM aspnet_users

However, i cant get it to work with a simple querystring...

<SelectParameters>
<asp:QueryStringParameter Name="username" QueryStringField="user" Type="String" />
</SelectParameters>

I want to fetch a users First and last name with a querystring, for example (default.aspx?user=DemoUser) But it doesn't work, it displays the latest entry in the aspnet_profile table. Btw, im trying to display it in a formview.