People forget about the amount of data LinkedIn has available to them: ConnectedHQ (the predecessor to LinkedIn Contacts) has direct access to thousands of email inboxes, Rapportive can log any time someone hovers over a new address, and millions of connections have been gleaned through 'import your contacts'.
If you really want to talk about creepy, I'm fairly sure they use your IP address to match against other people who live/work at the same location: when I created a test account with dummy information, the first contacts that were suggested to me were my roommates.
If you really want to talk about creepy, I'm fairly sure they use your IP address to match against other people who live/work at the same location: when I created a test account with dummy information, the first contacts that were suggested to me were my roommates.