Here are six of my favorite social networking services.
WAYN. This advertises itself as the 'facebook of travel'. The main advantage is that WAYN asks 'What are you UP for doing?' This is more social than the preening that goes on elsewhere and WAYN is a far better way to meet people.
LinkedIn is a sort of Facebook for business. Members create an account and post their C.V and qualifications. This grows larger and larger (associations, clubs, business associations and so on) and more and more business connections are made.
Plancast. This service brings together all of my social networks and planned events and presents them on a single profile so I can see the options open up me. The options I choose will then be displayed on my public profile for others to see and join.
43 Things is an interactive 'to do' list by which people list their life goals. Thankfully very few people post their shopping lists online although some people do. In theory this should be a useful business tool where people unite over common projects and aims. In reality it is more like a public a public set of new years resolutions where people cheer one another on.
Couchsurfing is an association of people who allow visitors- usually from other countries- to sleep on their couch or floor for a few days. It is a wonderful way to meet people as couchsurfers are friendly people.
Motley Fool is a finance website, properly speaking. It does have an active message board and the occasional social event so has a social networking character as well. I include this because it is dedicated to investment and financial survival. All other social activity cost you money but sorting out your finances will do the opposite.