BuddyPress.org

For Blog postings, you might try Glenn Slaven’s plugin. I haven’t tried it with WP 3.0.

https://wordpress.org/extend/plugins/friendfeed-comments/

You could also go with Disqus or Intense Debate … to synch comments in blog postings.

For activity stream … I have not tried this … but … use tweestream to sync between site and twitter, use FriendFeed tool to keep twitter and FriendFeed synchronized … that should do the trick at the basic level.