In my mind, the MU theme should gather all posts and comments as its own. I use two plugins to implement these function.

1.Sitewide recent comments

2.Sitewide Recent Posts for WordPress MU

Of course, the two plugin actually like API lib, you should know some PHP. Don’t worry, it’s so easy. The only thing you should do is exchanging sth with others.

OK, this is my final result, all 3p-blog members collected in the main page.