I like the idea of second accounts... in fact I have "second" Facebook and Gmail accounts. One is really me, the second set is what I use to sign-up for stuff I'm not sure I really want, spammy sites, etc.
If I were to be detained, I could just give them access to my second set of Fb and Gm accounts and be on my way.
Besides, if I were doing anything that I don't want to be linked to, why the heck would I use an email account easily traced back to me? You could easily establish some sort of algorithm of changing email addresses and use a service like Mailinator. So today I would check email@example.com, tomorrow firstname.lastname@example.org, etc. Could even be a random number generator such as Google Authenticator.