I joined LinkedIn a few months ago. It sold itself as an upmarket kind of place for professional people so I thought I'd give it a go. I didn't register myself with any special interests, just gave the minimum needed.
Almost immediately, my e-mail inbox was hit by spam from people who wanted me to read about their research and attend their conferences; and one man who wanted me tell me about his sponsored charity walk. I've told the spam filter to mark all LinkedIn mail as spam.