One of the major problems with social networks - hey, one of the major problems with any new trend - is that so many people spend so long trying to persuade themselves that they're actually looking at just another version of an existing thing. So if I'd been around when television was invented I wouldn't have been surprised to see so many people claiming it was the end of radio, and so on.
What's prompted this is the BBC's story about email being supplanted by social networks. Well, no it's not. Email is a simple one to one communication usually. It can of course be done en masse but even then, I'd hate to get a whole press release through Facebook (and couldn't get that through Twitter if I tried).
The thing about the new media is that - wait for it - they are new. They are therefore evolving and I have no doubt they'll find their niche, whether that niche exists already or not. Like TV not demolishing radio but both media realigning themselves because of the existence of the other, the one isn't going to replace the other. I'd be very interested in seeing comments from anyone who actually uses social media instead of mail - but I'd be really surprised if someone were actually doing so, every time.