I guess it may depend on the professional sphere that you are in. I don’t know of anyone who is on it or at least nobody I know has ever mentioned it to me.
At my company (an international IT systems company) management actually strongly encourages everyone to have a professional LinkedIn profile - they’ve even hosted training sessions on how to make a good profile, and they offer free professional memberships. We do it because we expect our customers to look us up (and vice versa) and the company as a whole wants to ensure its people present themselves well.
Almost everyone in my office (which includes finance, marketing, engineers, etc) is on LinkedIn. I know that because I am connected to them. S (who is in college) has a LinkedIn profile.
Having a strong LinkedIn profile is essential. Many companies aren’t willing to pay a headhunter’s commission because they simply don’t have to do so. Executive recruiters (high level headhunters) are used for very high level positions- C Suite type jobs. Otherwise, most companies use internal recruiters and LinkedIn.
Linkedin is valuable, though you have to be careful about people connecting to you (I get a ton of requests from people that I don’t know, who obviously are spamming trying to get people in certain industries), there is a lot on linked in, there are pretty good articles about job hunting, and they also will send you job recommendations based on your profile. If you are thinking of searching for a job, having a profile is huge. Plus linkedin can be a network to use when job hunting, among those you are connected to.
With headhunters, I always recommend talking to friends and colleagues in the same field you are. There are true recruiting firms that get hired by companies for the search for a specific job opening, other companies keep a group of firms on their ‘in list’ when they are hiring. There are also a lot of ‘bucket shops’ out there, that basically take your resume and spam it to open reqs they see, and to be honest stay away from them, it is why talking to friends is important, spammed resumes like that have about as much chance of being actually brought in is very, very small. You will know the difference, if they just take your resume and say “We’ll get back to you”, they likely are a spammer, good recruiters help you do things like tailor the resume to the job, explain about the person you would be talking to and so forth.