What I might consider the least social of the social networks out there today is LinkedIn. It may be however, your most important one. LinkedIn is a social media site where you connect with other people, mostly on a professional level. It is advised that outside of day-to-day people like family and close friends, that LinkedIn be used for people to give references and testimonials on your business relationship with them.
If you cannot vouch for the quality of their work or services, you should consider not being connected on LinkedIn with them. LinkedIn is becoming an online place to demonstrate and get professional references when job hunting and/or when determining whether or not to conduct business with someone. They also have a pretty impressive and highly qualified job search engine, along with showing you people you may be connected to at that company. This is unlike any other job searching website out there.
If Twitter is your soap box, and Facebook is your scrapbook and watering hole, consider LinkedIn your online rolodex and business card holder.