In those days, some of my colleagues were on ICQ or AOL but some were on Yahoo Messenger but some were on MSN but some had started to move to Skype etc. And that meant a lot of people had to have accounts with all of them because of course you can't control which of those systems the person you need to speak to likes to use. And tools like Kopete would spring up to help you deal with your many different messaging accounts.
It's pure speculation, but I think that's the direction I see the major networks going in. Already there are people who are Facebook friends whose Facebook status updates come from their Twitter app. Meanwhile many Twitter posts are there to point me to blog articles on blogs that I could also individually follow using RSS and Google Reader. And the rise of those social communities hasn't, for instance, stopped me from sometimes needing to use much older forms of community: forums, mailing lists, even (for a course I've been teaching) newsgroups.
So one more social network does not necessarily mean death to the rest. At the moment, I don't see Twitter and Facebook following Bebo and MySpace into relative insignificance. Instead to me it means another system I'll need to have an account on
(Edit: Thank you, I am now on Google+. Dominic, I can't seem to message you to say I'd already accepted an invitation just before yours came through; thanks for repeatedly inviting me, you are in my circles.)