Being in the UK, I use Facebook basically everyday to connect with my family and friends in Egypt. I added some of my students who are working now in different software companies worldwide. I don't think my Facebook account is representing my professional career, though I'm using my real details and added some profession history on the profile.
I built an account on LinkedIN to be connected with my career friends and the professionals in my field. It is really helpful to present your CV, work history, and all this can be supported by recommendations from others. Doing the PhD in computer science, I used it to connect to people I may not be able to meet at all. Researchers and other PhD students in the same area of research can accept your invitation immediately or through others introducing you to them. Though, I didn't use it to look for a job, yet. In LinkedIN, people can endorse your skills and expertise that they know you have, or they worked with you before and you showed that skill at that time. I have more than 250 connections and they are increasing!
At the same time, you are not going to be guaranteed a job once you published your CV or connected to employers, it is just opening a new window on you that employers can use to learn more about who they are going to employ. It gives them the confidence that you can represent yourself as a professional (than others). It gives you the opportunity to express your ideas and reactions towards different parties in different situations that you may not be able to express publicly in the scene. Personally, I need more academic work to be added to my experience, as well as more research publications. These are perfect representatives for an academic like me.