It's not just who you know. It's also who they know, and what they know. Online social networks have the potential to connect you to a vast world of people and resources, and they've gone from fad to fact of business life. Generally speaking, social networking services connect your list of personal contacts to the lists and profiles of others, giving you a bigger Rolodex of potential associates. These networks are finding ever more ways to be useful for tasks like finding employees and sales prospects, tracking down expertise, spreading marketing messages, and gathering customer feedback. Here are six services worth knowing.

Best for...Finding professionals

What it is: A membership service through which 11 million people list work experience, references, and job goals. LinkedIn's search engine, which scans the profiles, is an excellent tool for recruiting and job hunting. It's aimed at individuals, though some companies use it.

What's cool: A jobs area gives companies a huge base of connected businesspeople to recruit from. An answers service, which allows the posting of business questions, has a start-up and small-business category.

Drawbacks: Network spam--people you don't know will ask to connect with you.

Price: The basic version is free. Premium versions offer features such as a greater number of introduction requests, fuller access to other people's profiles, and the ability to directly contact people who aren't connected to you. Plans range widely, from $60 to $2,000 a year (or $20 to $200 a month).

Best for...Looking good fast

What it is: An online service that hosts customized social networks that use your own branding and Web address. Small World builds it; you can use it to link employees or to turn customers into a social network so they can share ideas.

What's cool: Support for reviews, ratings, and a video gallery allow companies to build libraries of things such as customer-generated product demonstrations. For business use, "friends" can be called "contacts," and "comments" are "testimonials." You control what happens to customer data, reducing privacy issues.

Drawbacks: It's pricey, though cheaper than hiring IT staff to build and maintain a network.

Price: There's a $10,000 to $75,000 setup fee. Monthly hosting fees vary based on the size of the network; it typically falls between $500 and $3,000.

Best for...Marketing to Gen-Y

What it is: You know about MySpace. About 65 million people use it to create pages with personal pictures, blog entries, video clips, and links to the pages of their friends. Its size and the passion of its users make MySpace a good way to build buzz among consumers, especially younger ones. Bands and authors build pages looking to get linked to by MySpace members and featured on MySpace pages dedicated to music or artists.

What's cool: Even if you don't have a MySpace page, the company's partnership with Google allows placement of ads targeted to specific pages, interests, and searches. If you have a page, you control how it looks, and it's easy to post audio and video.

Drawbacks: It's easy to develop an ugly MySpace page. It takes time to maintain a good one--time you could be spending on your main website.

Price: Free

Best for...Mobile marketing

What it is: A message-posting service designed to let people send very short messages--140 characters or less. While many people use it for short-form blogging, marketers can use it to post quickie updates to customers and work groups can use it to keep tabs on what other members are doing.

What's cool: Free search engines, developed by third parties, let you type in your company name and see Twitterers talking about it. In addition to computers, Twitter also runs on cell phones, so customers or co-workers don't have to be at a desk to get or post messages.

Drawbacks: Twitter is still experimental, and its developers haven't done anything to tailor it for business use. Making it work requires building your own network of customers or clients.

Price: Free

Best for...Hearing customers

What it is: An online service that lets users rate and comment on local businesses. You can see what your customers think about you and engage with them.

What's cool: Yelp can help entrepreneurs move their real-world buzz to the Web by capturing it in writing. The feedback provided by reviews and ratings can be invaluable--and it's cheaper than running a survey. Business owners can engage customers directly. Yelp also offers a sponsorship program in which companies can pay for increased prominence in searches.

Drawbacks: You don't control the content; consumers can post whatever they like. Business sponsorships are currently available in only three cities: San Francisco, Chicago, and Los Angeles.

Price: Free to register. Sponsorships cost $250 to $2,000 a month.

Best for...Helping salespeople

What it is: Software that integrates with e-mail, CRM applications, and other corporate programs to produce a searchable web of the relationships that exist within and outside a company.

What's cool: Companies can use Visible Path's software to grease the rails for salespeople by giving them better introductions to potential clients--the theory being that someone in your company might well have a good relationship with someone at a potential client company. It even defines the closeness of contacts (who's one personal connection away, two away, and so on).

Drawbacks: There's still not a lot of hard data to verify that using social networks leads to more sales than traditional cold calling.

Price: The basic version is free; a version with added support and administration is $20 per subscriber per month.