[obiterdictum]

I think it's more likely that LinkedIn mobile app grabs your phone contacts, if you happen to give it permissions to do so.

I've noticed that the "People you may know" section started to contain faceless placeholder entries with emails from my address book (though, I'm not sure if/when I've given the iOS app the address book access).

[christoph]

I noticed that recently as well. Some of them actually say "X shared contacts" below them as well. I know for a fact some of these people are not on LinkedIn, so it's essentially building up shadow profiles and trying to get users to "invite" them to the website for them.