Yeah, I'd be curious to see that too. The only problem with that is that Facebook's popular because it's a sort of all-in-one. If you don't care about your online presence, it does everything in one place. Twitter handles a much more limited subset of those things.
The reason why is that it's the same people that seem to be sending out invites on all those networks, but somehow it is disjoint from facebook which I think pulled a lot of people in the myspace (formerly the same crowd that was on geocities) towards it as well as a whole slew of people that have just 'established' themselves on the web.
It would be very interesting to see how much overlap there is between those sites.