LinkedIn is number one site for professional networking with more than 277 million users. Although it is social media, it differs from Facebook and Twitter and has it's own unspoken rules of behavior. If you want to take a full advantage of the greatest business social network - you need to know the do's and
DON'T Do Anything That You Wouldn't Do In Person
This rule, actually, should be applied everywhere in social media. Nevertheless, do not send random invitations to people you haven't met. Do not treat LI like twitter or FB, because the number of your links doesn't matter - it is not a popularity contest.