Social Networking is a term that describes people connecting on the Internet.  How they connect takes various forms.  Social Networking websites fall in two main categories, Social and Business related. There are numerous sites that simply bring people together for a common interest. Tribe.net, Yahoo Groups and Google Groups fall in this category.  Their goals aren’t necessarily to enable networking (though that may very well happen), rather to create a forum for discussion of a common interest.  Social sites such as Friendster, and Orkut have an explicit goal of connecting people socially.  Orkut’s home page states:

We
are committed to providing an online meeting place where people can
socialize, make new acquaintances and find others who share their
interests.

The business networking sites advertise different agendas.  LinkedIn’s website states:

With the help of the people you know and trust…As you activate your connections you will be able to reach hundreds of thousands in every industry profession and location…to help you, Hire, Find a Job, Close a Deal, Find New Clients.

The common thread is that the Social Networking sites/services promise to do what you could not do alone, meet people that can help further your personal or professional goals. 

Social Networking as a business is meeting with mixed reviews.  Certainly it can and has worked for many people. For others, not so much.  This post links to two pessimistic reviews of Social Networking.

An insightful commenter offers this point on social networking:

I think the really successful web communities aren’t built for social networking
– they are created (and have evolved) for other purposes, and the social
networking is merely a byproduct, albeit an important one.

Of course you don’t need a special site to use the Internet to network.  Blogs can serve that purpose and often do by the way they naturally work.  People subscribe to other bloggers whose content they enjoy or benefit from.  Comments on posts form a threaded discussion.  Often the comments lead to new contacts.  The feature of sharing your blog-roll is a subtle endorsement of what others have to say.  Search services such as PubSub and Technorati with the ability to "subscribe" to various keywords is a very efficient method to connect to others over common topics of interest wherever the content and comments may be posted on the Internet.