Sites like Linked-in are there to get the first initial contact. They're used by losers and sad gits who can't get interviews through conventional means to try a put out a wide trawl out in the hope some recruiter somewhere will see it and call
Of course that means you can only appeal to a certain class of recruiter: those who specialise in wide-boy sales / marketing roles and haven't got a clue about quality, skills and abilities
The down side is that by using such sites you are burning your bridges with any contact with any reputable employers
Sorry but thats absolute rubbish and I am critical of Linkedin if just used as a Facebook for business
Sites like Linkedin as a way of weeding out some of the chaff plus allowing employers and agencies an opportunity to get the advert out there without spending £5k to national media plus 40% of salary to an Agency.
Unless you feel companies like Vodafone / Tesco / Sainsbury's / Shell / BT etc are not reputable .....
Its a great way of looking just who someone is connected to, as too many and you they a contact chaser and too few then maybe there is a reason.
In any medium sized org a recuiter may not know that X, who joined last year worked for B company 3 years ago, where potential interviewee coming from. A quick call as to whether interviewee is as capable as CV can save a lot of crap.
Agencies can have specific requirements that none of their candidates registered have, a search on Linkedin opens it a lot, otherwise where do you advertise, in the hope that someone with the requisite skills see it ?