Google+ is the fastest growing networking site…for businesses. While it is still small compared with LinkedIn, it is useful to test marcomm and social approaches to the target markets/communities you want to develop. For journalists, it is another key place to find communities.
Many entrepreneurs fail to understand twitter is much more than a site to tweet stuff, it is a place where communities form for their mutual benefit and experts connect with others in 1-to-1 conversations. You need to have a twitter entity for your company’s future.
Facebook is good for linking with people but may not be important for your applications. Companies need Facebook pages but it is optional to your professional persona.
Fast growing site. Why should content be text only? Pinterest is both fun & professional. It is becoming a key way to connect with a different type of social individual.
Digg is a passion community site where people discuss their passions and new trending topics with each other.
Another passion community site. It is more eclectic compared to Digg.