In the use of social networking, I would choose Facebook over any of the other sites. It allows me to connect in a personal way. I would keep LinkedIn and Plaxo solely on a professional level. We must differentiate between our social circle of friends and our professional circle of friends. At times these may over-lap, but on emust decide which site to use.
Facebook is more for meeting up with old friends. If one were to post pictures here, they could include pictures of their family and fun activities (more informal).
LinkedIn and Plaxo ask for information that is more for professional use. One would use these sites when looking for a new postion or to re-locate to a new area. Pictures one posts here should be of only themselves and looking professional more formal).