Check out www.subjot.com (reviewed here: http://thenextweb.com/apps/2011/08/11/subjot-a-twitter-alternative-that-lets-you-curate-content/ http://thenextweb.com/

Check out www.subjot.com (reviewed here: http://thenextweb.com/apps/2011/08/11/subjot-a-twitter-alternative-that-lets-you-curate-content/ http://thenextweb.com/apps/2011/08/11/subjot-a-twitter-alter...) for an implementation that lets users subscribe to friends for only certain topics. I suspect it only works with public posts, though (but haven't used the service to know for sure).