It's super-creepy. All you need is someone who has previously emailed or chatted to you via e.g. gmail to give their email address and password to LinkedIn to "find contacts". The seem to go about mining a graph they have constructed using various people's email correspondence/address books, even including contacts via a nameless email addresses you never provided to LinkedIn.
When I signed up I saw a bunch of people I had as contacts on a gmail account which I hardly ever use. I had NOT used said gmail account when signing up for LinkedIn (and no LinkedIn app etc.). They seemed to have somehow associated me with contacts for an email address I had told them nothing about by piecing together the contact lists of the small number of contacts I added on LinkedIn and linked the name in their address books to my unnamed gmail account. I simply don't trust them anymore.
When I signed up I saw a bunch of people I had as contacts on a gmail account which I hardly ever use. I had NOT used said gmail account when signing up for LinkedIn (and no LinkedIn app etc.). They seemed to have somehow associated me with contacts for an email address I had told them nothing about by piecing together the contact lists of the small number of contacts I added on LinkedIn and linked the name in their address books to my unnamed gmail account. I simply don't trust them anymore.