Linked-In not really Opt-in?
Recently there was some kerfuffle about LinkedIn silently changing privacy settings. In fact, they didn't do that "silently", nor was it "recent". Remember that large chunk of text nobody reads with the "Agree" button you clicked on? Those were the new terms, and deeply buried in them was this privacy settings stuff. LinkedIn's Eric Heath blogged about that, bringing it as "more control over your LinkedIn information". Sadly, the default setting for this new "control" is "rather open". But you can change that.
If you want to be sure that LinkedIn does not use your profile in ads in the future ever again, you could always cancel your account. For people that don't want to cancel their account, there is a setting you can try to change (given that LinkedIn doesn't change it again). Here's a little screenshot I annotated with Skitch:
You might also want to check out some other settings on those tabs. There's no clear overview, and the settings are hard to click through. That's probably on purpose, because well, Linked-In is now cashing in on it's (or: your) data.
Remember: What's not on the internet, is not on the internet.