Ask HN: How important is linkedin and other social networks for recruiters?

I am scared of social networks, I don't trust any of them but recently I atended a conference and some HR people gave an huge importance to Linkedin, how much does someone without an account miss? Are recruiters going to worry that I have no presence in them? Am I going to be asked about it?

Is it worth to sacrificy privacy to get a job? I guess that last question is up to each one.