I always assumed the suggestions were from other people importing their Twitters and email contacts. So if person A emails me and then imports their emails, Linkedin can suggest to both of us that we might know each other, increasing the chance of creating a connection (and creepy-ness factor for me).
I'm pretty sure that's the case. I got suggestions for 2 people I played WoW with and the only ways we communicated were in-game chat, voice chat, and we had a single e-mail chain where we (the officers) talked about....something, I forget what. I'm guessing one of them imported their mail contacts because I doubt LinkedIn has visibility into Blizzard's social network or private voice chat servers.
I think the creepy part about LinkedIn is you can actually see who views who. If Facebook did that it would be very embarrassing for a lot of people.
Twitter is more interesting in this case because it allows unilateral following. So even if you broke it off with a now ex-girlfriend if they were still (or even only now) following you on Twitter that graph might make indicate the awareness.
The automatic links are one of the reasons I don't use LinkedIn. I have worked at places with people that I do not under any circumstances want to be associated with. I work in a completely different city now and will be moving to another town, which will make as clean a break from them as possible. A social network continuing to track that is a significant issue.
