- aggregated feeds from outside sources
- significant personal / professional profile information available from members
- large networks of friends to draw from
- events, and groups to participate in
- conversation threads
- fine-grained privacy control
- a large audience already acquired
… and the list goes on. As others have noticed, it's a killer combination.
Yes, it's true that Facebook can't deliver updates to my IM client the way that Twitter can (or at least, I haven't found that feature yet), but I am personally about to turn that feature on Twitter off. While it's interesting to get a real time stream of tweets from my friend, it's also a disturbance during meetings, and actually interferes with getting real work done on the mobile phone.