I started using LinkedIn at my corporate job a few years ago. Our company was being acquired by a national wireless telecommunications provider and everyone started connecting on LinkedIn before we got booted. I have 100+ contacts right now that consist of past co-workers, clients, fellow VAs, and potential clients that I've talked to.
I've also joined some groups and I receive weekly digest emails so I know if there is something I can respond to.
To me LinkedIn is a way to stay connected and to share your knowledge. I highly recommend it.

Holly