I was always a skeptic on social networking until a few years ago while working at R/GA I started using Facebook to be more versatile in the “medium” for my clients. And let me tell you – it’s incredible, really incredible how fabulous social networking is on many levels – keeping up with colleagues/old friends/acquaintances, etc. As a small or large business, having a “fan” page on Facebook is essential and a great way to have true ambassadors for your company or brand; plus it is truly easy to set up and free (plus additional ads on these sites are affordable and pretty precisely targeted) . I also love LinkedIn for the many opportunities that the site presents for networking, job opportunities, career education, etc. I have used ASmallWorld which is great for Travel advice, city hot spot tips, etc. These sites are — POWERFUL. I just started Twittering recently, so I will tell you how that goes … Social networks should definitely be in your arsenal of strategies for the best (and affordable) digital marketing opportunities for your business.
Tip: Ning (http://www.ning.com/) lets you set up your own social network for free.
Wikipedia has a good list of social networks and how many users they have: