Yup, I'm one of those people.

I don't use Facebook, and it's unreasonable to expect me to sign up for a Facebook account so that I can use your service.

I do use Twitter, but I'd rather not share my account details (even just the username) with someone I've just met.

I've always thought that being able to explore the web of information out there about me was kind of creepy, and I'm not about to make it easier for someone to do by creating connections between the accounts I have with disparate service providers. I like when my data is siloed, and only shared on my terms, with my explicit consent.