Social Networking Software - a future for e-mail?

I was just talking to my new friend Sylvia about the social network craze and how it is much more widespread in her native country, the Philippines. And the whole thing was triggered by her asking me, if I know Friendster. And I quite liked her description of the whole thing:

it's like e-mail only a lil more different than e-mail

And it turned out, that she is using Friendster like a webmail client to send spam-free messages and emails to her friends, who are in Friendster as well. I didn't consider this use for SNS yet, but I quite like the idea.