I haven't really understood the popularity of social networking tools until now. I did create profiles on Friendster, Linkedin, and My Space and I do like how I can see what my colleagues are up to on Linkedin and connect with all my High School friends on My Space, but aside from that I haven't make much effort to "add friends or colleagues" or use all the system features.
Recently I saw a demo of Facebook and all its capabilities. Wow!! Facebook is so different then these other tools because of all the awesome applications to choose from. I can bring my blog, my flickr photos, books I read, my delicious tags, the places I've traveled, and much more all into one page and easily move around each of those elements. Basically my personal life and my professional life merge on this one page.
I also really like how nonprofits can create applications in Facebook (see example) to get new supporters and raise awareness of their cause. This is a great tool for less web savvy folks or groups to have their own page on the web.