I use LinkedIn since several years, it allows you to connect your professional profile to a lot of people in millions of companies around the world. Depending on what you do, being and get connected in LinkedIn with all the people you’ve worked with and know professionally can open you doors in the present and in the future.
One thing LinkedIn does is to suggest you “People you may know” to add them to your own network, and its this spec which I am not so happy with.
Some weeks ago I checked my profile and added some contacts that I know, I was surprised they were on LinkedIn so I add them as suggested by the site. My surprise was when I received an email from them saying they don’t use LinkedIn at all, then I realized, LinkedIn was sending invitations to these contacts so they register in LinkedIn!!!