Social networking is no longer the Next Big Thing; it's now as much part of our Web experience as search engines. Previously considered the province of kids who wanted to keep up with class gossip, social networking services are being co-opted by grownups who are examining ways to use them both within and outside of their places of employment.

At least one social networking site, LinkedIn, has been vying for an adult usership since its introduction in 2003. LinkedIn allows users to create and maintain a list of their professional contacts (and friends as well); the purpose is to be able to network - to have access to your contacts' contacts, and in that way further your professional outlook. You want to find a job? A new sales opportunity? Information about a client? Here's a way to do that.

LinkedIn has remained remarkably stable in its services. It has made some concessions to Web 2.0 expectations by adding a job board as well as areas where you can find recommendations for service providers or answers to questions. It also offers premium services that allow users to access more information and the ability to contact second- or third-degree contacts (in other words, friends of friends of friends). However, it has not swerved from its original mission: to be a business-only service rather than a more generalized social networking site.

There are few other sites that are as focused as LinkedIn, but at least one has moved from being only for socializing to being a business tool as well. Facebook began in 2004 as a site for US university students - ie, people with university e-mail addresses - to socialize online, and was only opened to the general public in 2006. Since then, it has rivaled MySpace as the place to hang out, but it has also attracted an increasingly adult audience who want to use it as a means to discuss their professions rather than their latest crushes.

Facebook has a much wider range of services than LinkedIn - mainly because of the large number of third-party applications that people can install and use - and so it is a more flexible medium of communication. It may also be an advantage to companies to use a service that employees are probably already familiar with.

However, the use of Facebook as a means for business networking has been controversial. Companies that want to use Facebook to keep their increasingly mobile employees in touch are concerned - not without reason - that all those games, social groups and quizzes ("Can you name the Muppet characters?") can distract people from actually doing work.

So which is actually better for professional use, by both companies and employees (and would-be employees): LinkedIn with its focused approach, or Facebook with its multitude of applications?

To check this out, we came up with six familiar business scenarios and asked two of our writers to solve them: one by using Facebook, and one by using LinkedIn. Sometimes it was clear which service could do the job better, but sometimes it was difficult, or impossible, to choose a winner.

It turned out, in fact, that there is no absolute winner; both Facebook and LinkedIn excel in different scenarios. It all depends on what you need to do.

Which would you choose? Check out our six business scenarios, and see what you think. You may decide to pick one over the other, or may simply decide to join both. - Barbara Krasnoff