What Hook do I need to use??

I am creating a plugin for my WPMS that needs to change user meta data when a new user activates the site from the link sent by WP when the user signs up for a blog.

add_action('wpmu_activate_user','lf_wpmu_activate_user',10,3);    

function lf_wpmu_activate_user($user_id){ 

    $info = get_userdata($user_id);

    $args = array(
        'ID'=>$user_id,
        'test_meta_data' =>'this is a test',
        'show_welcome_panel' => 0

    );
    wp_update_user($args);

}

I can verify that the code is being read by WP with a few lines of code that writes to a file. But my function is not being fired when a user clicks on the link to activate the site.

Am I hooking to the correct hook? Or should I be connecting to a different hook.. this seems like it should be fairly simple but it is not working.

Thanks for the help..