If not, go check it out. I am learning it is an absolute must when you are looking for a job. I got on, estalished my profile, and did an inquiry on who else I know on LinkedIn! Good golly you would be amazed. They say today that 80% of the jobs people find are done so through networking! Check it out. I am also told headhunters and recruiters are always looking in there too for potential matches!
For everyone else, even if you are not job hunting, it is a great way to stay connected to colleagues (old and current), your buddies from HS or college. It makes a ton of sense.