I agree. My personal anecdote is that I've been to three countries in the past year; Japan, Mexico and Colombia; and later connected with every single person I met that was in their teens or twenties. This includes some fairly rural parts of Mexico and Colombia that don't necessarily have broadband or very good Internet access, in general.
From what I noticed among my japanese friends however, Mixi is a lot more popular over there than Facebook. It's less open though: invitation-only, and you need a japanese cellphone address to register, so I've seen few travelers/foreigners on it.
It's pretty geared towards Japanese cultural norms, from what I've read and what my own Japanese friends have told me. I don't think it would be as acceptable as it is here to meet someone briefly through a friend and then add them the next day on facebook, for instance.
It's an interesting barrier towards global adoption, methinks; it's like how neither Google nor Yahoo can beat Baidu in mainland China, because Baidu just understands what its audience wants better. But, I think that a social networking platform is going to struggle a lot more with adapting to cultural norms than information search is.
