try use this function,,
function get_primary_blog_url($id=null) {
// is there a user to check?
global $current_user;
$user_id = (null != $id ) ? $id : $current_user->ID;
if ( isset( $user_id ) ) {
// Get the blogs of this user
$user_blogs = get_blogs_of_user( $user_id );
// Get the URL of the blog
foreach ( $user_blogs AS $user_blog ) {
if ( $user_blog->userblog_id != BLOG_ID_CURRENT_SITE ) {
return $user_blog->siteurl;
}
}
// If user has no blog, return to the homepage.
} else {
return home_url();
}
}