LinkedIn

[Bibby]

Basically, LinkedIn is a public-visible resume, and it's a tool for staying in touch with your wider circle of colleagues. It's also a place to publish articles of varying quality, and some people use LinkedIn groups to discuss topics of interest. I've had a few recruiters contact me on LinkedIn about job opportunities (although in every case, it was either a job I didn't want, or else they ended up hiring someone with more experience).

If you're a recent graduate looking for a job, I highly recommend having a LinkedIn account and making sure your profile is complete and up-to-date. It won't hurt you to have a professional online presence, and it may help you find a promising opportunity. On the other hand, if you're settled in your career, and you're not using LinkedIn for professional networking or publishing articles, making a LinkedIn might not help you very much.

As Sanantone mentioned, LinkedIn is valuable in some fields but practically useless in others. It's big in the IT, finance, and marketing fields. It's probably less useful in healthcare, security, elementary education, or law (with the possible exception of certain highly specialized positions in those fields).