LinkedIn may not be entirely dead, but in the past several months, I've received nothing but "business friend" requests from sales people and consultants trolling for business. LinkedIn has enabled "lazy" sales work. Just browse LinkedIn for keyword matches for whatever you are selling, and then send a "link" request. I turn them all down.
I was a very early LinkedIn user, and have yet to find it particularly useful. I've never let it have access to my address book, and I rarely use it reach out to people I already know--email and the phone are far more efficient.
LinkedIn is basically Facebook for business people, and aside from the news feeds, it does not offer me much functionality.