Evan Doll, speaking at the Kellogg PE/VC conference, made an interesting distinction between "friend networks" and "taste networks". Facebook is a friend network. If you "unfriend" someone on Facebook, it can have social consequences in your real life. This is because to "friend" someone requires permission from both parties for the connection to be made.
In contrast, Twitter is much more a "taste network". It better reflects people you are following to get a feed of information related to your tastes. This is partly a reflection of the one-sided permission model; the other party does not need to accept you as a friend. This allows you to follow celebrities and others, who have have interesting things to say, but might not necessarily want to "friend" everyone.
Evan Doll founded Flipboard, which neatly brings content flowing though all your social networks together in a fun "social magazine".