LinkedIn is also a frequently used sock puppet for identity thieves. Never accept connections from people you don't know. Never put your real physical address into the contact section; give an e-mail address and an unlisted telephone number.
LinkedIn is also a tool that many recruiters (scalpers/headhunters) use in a dishonest manner. Their attempts to connect with you are phishing expeditions to get your connection list. The job that they claim to have is nearly always fake; it is bait, if you accept their connection you will never hear from them again or they will tell you that the job is filled. Recruiters always hide all their connections so you get nothing from them (all take and no give)
Ignore any offer for connections from a recruiter. If you want to contact them, do so outside of LinkedIn. Read their public profile (many recruiters have contact information there). If so, call or Email directly (use unlisted phone). If not, send an InMessage through LinkedIn giving them only an Email address and an unlisted phone number.
2. A profile graveyard for entry level corporate kids who do not want to share their Facebook profile with their superiors.
3. A place where salesmen and MLM marketers pump their wares until they realize no-one ever returns or uses LinkedIn.
Jane: "I'm ok, still looking for a job."
Bob: "You should go on LinkedIn, it's like a Facebook for professionals."
Jane: "Thanks, I'll check it out. My kids keep talking about social media."