I haven't heard that. I just assume that there are enough people out there that I've conversed with that have uploaded their own contact list to LinkedIn to find connections.

Gmail has/had that behavior where anyone you email ends up getting created as a contact, and in the early days it seemed a lot more aggressive as to when it would auto-add someone to the list. So there are a lot of people who have spurious entries in their contact lists.

I know it wasn't worth the time to me to clean the list, up until I bought an Android phone in 2010. So I had six years worth of auto-added contacts, going back to when I first got a gmail invite.