Facebook, Google or Twitter, because I don't think those companies know enough about me yet.
Sane people use Abine's Maskme things they think they might want to use and Mailinator or one of the other disposables places for those they doubt they'll use.
Email, because I don't feel Facebook/Google/Twitter gives me the ownership of my virtual identity.
Email with a throwaway address. I don't use Facebook, I don't trust Twitter, I don't expose my Google account, and I don't trust a new product with my real email addresses.