With multiple identities, on the other hand, I could have my real identity, with my real name, and then - for example - my hacker identity, or my secretly gay identity or whatever. With the guarantee that for other users there is absolutely no way to connect the dots if I don't want that myself, but with an easy to use interface that allows me to manage them all as seamlessly as possible.
I personally wouldn't mind if the provider was able to connect the dots and use that information for targeted advertising - as long as it doesn't sell my identity to the advertisers, only the target. And this could become a real differentiator from FB.
By the way, my wife already manages different identities by using different services (ie facebook and linkedin).
About resharing, of course if you use the same real name in both services that would't be different from circles - it would just make it much harder to reshare, but by no way impossible. That's not my wife's use case. But if you use different names, tracing you becomes pretty difficult.
facebook does not allow multiple accounts.
twitter doesn't have that many users.