I have used LinkedIn for so long that I don’t recall when I started – around 2003. I have long since forgotten who first invited me to join it. But I immediately recognized its qualities and have regularly invested time in it ever since.
I have over 525 connections, and know each person individually with rare exception. I have accepted connection requests for three recruiters who are well-connected; otherwise, I deny all link requests from persons I do not know personally.
LinkedIn has opened doors for me, helped people find me, and helped me find people. LinkedIn is a great tool for locating subject matter experts, former colleagues, and (I hear) jobs.
My public LinkedIn profile is here: