Meant to add I once had to log into Facebook - someone in America signed up to it and made a mistake in their email address.
I got the Welcome email, followed by another that read like a double optin email.
But then I started to get email from the persons friends.
So - it looks like you can sign up, get a double optin email, but sill be able to login without doing the link in the double optin email (since I got the email, how could they have clicked the link?!!!)
So, getting fedup with the emails, I did a password change request, logged in, sent a message to their daughter explaining that they had used my email address, and that I would delete the Facebook account - which I did!.
I then got a few password request emails but no other emails from the site.
Then I got a conference booking confirmation email! It had someone elses email also listed on it.
I contacted the person and politely asked the to get the other person to make sure they used the corect email address.
To say I got stroppy emails back "how are you using their address" "I know how long they've had this address" and finally "I'll report you to"
At this point I replied "go ahead, and by the way look at the date of a screenshot of my earliest emails from my email address"
Still waiting for a sorry!!!