BuddyPress.org

Best way to do this would be to integrate the P2 theme into your user blogs.

Ahh yes! Totally forgot about the P2 theme, John!

https://wordpress.org/extend/themes/p2

I forget if P2 puts a character limit on post entries like Twitter.

—

Also I did some digging just now and the WPMU plugin that John and I are talking about is the “Menus” plugin by dsader:

http://wpmudev.org/project/Menus