It's not what you know, but whom you know - and whom they know, and whom their friends know, and who their friends' friends know. That's the idea behind the growing number of social-networking communities.

In many ways, these Web-based gathering places - such as LinkedIn and ReferNet - couldn't be more dissimilar. Some let you build an electronic address book and link it to similar lists set up by people you know and, by extension, to people they know; all of you can then search that large Web of contacts. Some allow you to create a private networking group that you can invite others to join; others only allow you to participate in existing discussion forums. There are sites that are strictly social, intended for fun or romance, while others are all business. Most are free, though a few charge fees for subscriptions, premium services, or software.

Still, all true networking sites share two basic principles:

They make introductions. They help you meet people you don't know through people you do, much as you might at a cocktail party. Or, they drop you into a network of people with a common affiliation or interest, much like joining an alumni association, trade or professional group, or local business club. Many networking sites - most notably, MeetUp - hold in-person gatherings in addition to connecting people online.

They rely on trust. Social-networking sites are not intended to let you link randomly with strangers worldwide. Instead, they focus on making connections based on your existing online and offline relationships - that is, getting people you know to link you to other people you might want to know.

While social networking sites are a 21st-century phenomenon, the underlying concept dates back to the 19th century. That's when Italian physicist Guglielmo Marconi advanced the "six degrees of separation" theory, the idea that anyone on Earth can be connected to anyone else through a chain of six or fewer people. In the 1960s, Harvard social psychologist Stanley Milgram tested the idea in an experiment using chain letters that typically reached their intended targets after six transactions. In the 1980s and 1990s, the phrase came to denote a movie, a play, and an informal game that challenged players to link the actor Kevin Bacon to anybody else in Hollywood in - you guessed it - no more than six steps.

Today, social-networking sites allow you to find the expertise you need in just a few degrees -- or links -- from your own circle of contacts. For instance, let's say you need a contract Web designer with a specific set of skills, but you don't know anybody who fits the bill. You ask your friends, but it turns out they don't know personally know any Web designers, either. So you go out one or two more circles, asking your friends' friends and then their friends for a Web designer referral. Chances are that somewhere out in that extended network, somebody knows just the right person. And if that distant affiliation isn't as ideal as a first-hand referral, at least there's still a trusted trail back to the source of the recommendation.

Good theory, but does it work in practice? Absolutely, according to entrepreneurs who have used networking sites to find employees, identify sales leads, and connect with potential customers. LinkedIn - the best-known of the business sites, which claims to have 4,000 CEOs among its 250,000 registered users -- processes more than 1,000 referrals each month. Nobody knows how many result in real business, but as Konstantin Guericke, LinkedIn's co-founder and vice president notes, every referral starts as an interaction between two people who already know each other.

Sometimes those interactions come with highly beneficial side effects. That's how it worked for a Jason Diaz, a Philadelphia-based entrepreneur who was looking for a payroll-software expert. With his LinkedIn connections, he quickly found a candidate who not only came highly recommended by known sources, but introduced Diaz to a big-company CEO Diaz wouldn't have met otherwise.

Industry heavyweights clearly view social-networking as more than this year's fad. Microsoft, Google, and Yahoo!, among others, have all launched or invested heavily in social-networking initiatives.

There are, of course, downsides. Some businesspeople steer clear of online networking because of concerns about compromising their own privacy - or that of their contacts - by revealing too much personal information. Others report that their e-mail spam levels jump markedly as soon as their contact information goes online. Another concern is a landscape still in flux. Forrester Research warns that users should expect some significant consolidation among major business- and social-networking sites over the next year or so.

For now, though, the benefits of being able to quickly access webs of valuable contacts outweigh those concerns for many users. The way they see it, such worries are a small trade-off for the chance to collapse six degrees of separation down to two. Or one. Or none.