LinkedIn is my friend. A good friend, in fact.
As a recruiter, I've found it's a wonderful tool to use to dig around and find all sorts of leads. It's six degrees of separation at its finest. I know that it's grown at an incredible rate over the last few years, meaning lots of folks have come to value it as much as I have.
My first impression years ago was that it'd be a business-y site. A place where engineers and developers and "business" people would connect. By that I mean, non-creatives. So happy now that our industry has embraced it as well. I can search art directors or writers, narrow by location and find a slew of potential candidates. The great thing from there is I can find others who are connections of these candidates, and go deeper and deeper into their networks.
Not sure about other recruiters, as for me, I am on LinkedIn everyday. I look at the news feed, I check open jobs (see if any competitors around me are hiring), I see who it suggests I might want to connect with. And, I search for candidates. It's not always for active, open jobs but I do like to keep ahead of the curve and get a few names for future reference.
I have noticed one thing that surprises me. I see a huge percentage of creatives that don't have links to their portfolio on their profile. My advice: you should, you should, you absolutely should. I've also seen a chunk of creatives who have a link on their profile that says "my company" and when you click it is their employer's website. Do you really need to be promoting them instead of you?
If you know how many people (read= recruiters) troll around on LinkedIn, you'll then appreciate the value of having your portfolio at the ready, even if you aren't looking for a new job. Don't pass on any opportunity to get your name and work out there, someday it'll be worth it.