LinkedIn is a professional networking site that allow users to do most of those things you could do on any social network, but in a professional atmosphere. My favorite feature is the ability to get recommended from past coworkers or bosses online. There are also a multitude of jobs from which you can search. It's also useful for checked up on that local vet or OBGYN for references. Even though there are a lot of people on LinkedIn, you'll find yourself wanting more. Of course, you could always make it your personal mission to add more people, but it's going to be a while before I use it on a day to day basis.