When it comes to finding a new job, they say it’s all about who you know. With the rise of online social networks that has never been truer.

Today, 42% of adults in the U.S. with Internet access maintain a profile on a social networking site, up from 20% in 2007, according to Forrester Research. And in an economy where almost one-tenth of the population is unemployed, more job-seekers are likely to look for opportunities online.

Meanwhile existing members of social networks may take the time to fill in more of their job history in their profiles.

Recruiters have been scouring professionally-oriented social network LinkedIn for qualified candidates for years now. More than 40% of Fortune 100 companies pay to use the site to find talent among its 46 million members.

But social networks are still evolving as places to hire and be hired, and Facebook, with its 250 million members, is gaining ground.

Unlike its more career-focused competitor, Facebook offers members profiles that tend to reflect their whole life. In the past that deterred many who were concerned an incriminating photo or wall post might be discovered by a potential new boss.

Professional and personal lines blurring

That fear is going away as people become more comfortable sharing their lives online, sometimes even blending their personal and professional personas. Some users take advantage of Facebook’s privacy settings to edit the information they present to professional contacts.

More importantly, Facebook is gaining credibility as a tool for recruiters and human resources professionals, the very folks who’ve been avid fans of LinkedIn. For one thing, Facebook seems to cast a wider net and provide recruiters with more references – and more outlets to spread the news about a job opening. Facebook users have an average of 120 friends. While LinkedIn won’t release this statistic for its members, recruiters say the average number of connections likely is smaller because of the site’s narrower scope.

The types of relationships and contacts found in a personal network versus a professional one are also assumed to be more authentic and less transactional, and therefore more desirable to marketers. And while Facebook members now span all ages and demographics, the average age of its users is 31, compared to 41-years-old for LinkedIn.

These are aspects that attracted technology company EMC to Facebook. “Many college grads aren’t on LinkedIn,” says Polly Pearson, VP of employment brand and strategy engagement at EMC. “We’re going where target market is—that’s why we’re on Facebook.”

Reaching recent college grads

As a business-to-business lacking strong consumer awareness, EMC relies on its Facebook page to build its reputation as a great place to work. It’s still too early to judge the success of EMC’s foray into social networking—the page only has 745 fans—but Pearson says the company has made hires through social networks and is more focused on starting a dialogue with potential employees.

“We have to have a huge pipeline,” explains Pearson. “A company can have a website all day long but there’s a lot more people hanging out on Facebook than on our website.”

As a technology company, EMC took a DIY approach to Facebook. But companies like the Pepsi Bottling Group , Harley Davidson and Bally Fitness rely on CareerBuilder to develop a presence on social networks. The career site consults with 250 clients, helping them build Facebook community pages that incorporate job listings, blogs about applying for positions and discussions among candidates and employees.

“Companies are always initially afraid,” says CareerBuilder chief marketing officer Richard Castellini. “We’re trying to help them understand that by giving up control you’re still going to get benefits in terms of deeper and wider conversations.”

While active job seekers might visit the Facebook pages of companies they’re interested in, Facebook also enables companies to reach target candidates who might not be looking. This more recent advance is made possible by Facebook Connect, a feature that enables websites and applications to access information in a user’s profile (with their permission).

Two Silicon Valley start-ups are deploying the feature to help companies encourage their employees to refer friends for open positions. Internal referral candidates are the most appealing to human resources departments since they are less expensive to find and tend to be the best fit for a position, meaning less turnover.

San Francisco-based Jobvite manages the entire hiring process for its customers, which include Tivo , Mattson and Mozilla. The firm, which announced an additional $8.25 million in funding yesterday from ATA Ventures and CMEA Capital on top of an initial $7.2 million investment from CMEA in 2007, operates a “software as a service,” or SAAS, platform. Clients simply subscribe the service on a monthly basis, rather than paying to have the software installed on their own servers.

The name Jobvite originally referred to a feature that enabled clients’ employees to use Outlook to generate invitations to apply for jobs at their company. But that required employees to sift through their contacts themselves, assuming they even used Outlook as their source for email addresses.

In February the company created an application that used Facebook Connect to allow employees to send job invitations via Facebook. The app will even search the profiles of the employee’s Facebook friends to see who might make a good match for the job. The invite can then also be forwarded on to friends outside the original employee’s network. (The invite feature is also available for LinkedIn.)

“We help companies scale how they hire people through networks,” says Jobvite CEO Dan Finnigan, who spent five years at Yahoo’s HotJobs unit prior to joining the start-up last year.

Appirio in San Mateo offers a similar invite feature as software-as-a-service, but only for customers of Salesforce.com . The firm, which is backed by Salesforce, Sequoia Capital and GGV Capital, offers applications that manage referrals in social networks, whether for hiring purposes or a marketing campaign. All the data is sent back to Salesforce for analysis and tracking.

If you’re fortunate enough to have a job right now, especially one you love, all of this may not seem especially relevant to you. But at some point in your career, you could simply be posting a status update when your next job comes looking for you.