Having only been using Twitter for a little while, I don’t feel that I have yet fully defined what I use each one for.
Certainly more real-world friends are on FB than on Twitter so I tend to use FB as a “personal life” aggregator – so, for example, ideally when I upload photos to Flickr or Picasa something gets posted to FB. Likewise if I want to share something with friends, it gets linked to on FB.
I had to take a look at my twitter posts just now to see what I’m using twitter for, and it appears mostly to be contact with people outside my circle of actual, real-world, friends.
There’s no doubt however that using Twitter has brought rewards in terms of new acquaintances etc. That may be because there’s not a huge take-up of it here in my part of rural England so those who do use it seem to be good at finding each other and getting in touch at arms length as you can on Twitter.
I’m not yet using it for business purposes (although have grabbed our business name) but think it likely that we will at some point be using it to extend our reach.