Social media is a great thing and employers know it. That’s why so many companies are using services such as Twitter and Facebook to get the message out about their products and services; and also getting their own employees to tweet and post comments in an effort to promote their wares.

A reader recently wrote me that he uploaded something personal on his company Facebook page by mistake. He ended up deleting it immediately, but wondered what would have happened if he hadn’t caught his error.

What would have happened? That’s the big question. Employees are sort of in the dark on this because so many employers don’t even have policies regarding social media use. A new study of multi-national employers found that while 75 percent of companies use a host of social media tools, only about half have corporate policies regarding that use by employees.

The survey, done by one of the nation’s leading labor law firms Proskauer, also found that among those employers polled, 40 percent said employees misused social media.

“The business world is experiencing an ongoing and rapid proliferation in the use of social networking,” said Daniel Ornstein, a partner with the firm. “Although this has many benefits, hardly a week goes by without yet another new story of a viral tweet or posting that has the potential to damage the reputation of a business, underscoring the need for companies to be proactive in this area.”

Here are some key findings:

* More than three-quarters of responders use social networking for business.
* The majority of these have only started doing so in the last two years.
* Just over a quarter of responders actively block employees’ access to social networking
sites, and a similar percentage monitor employee use of social networking sites.
* Nearly 40 percent of the businesses in the survey have had to deal with employees
misusing social networks, and nearly a third have had to take disciplinary action against
employees in relation to misuse of social networks.
* Despite the widespread use and misuse of social networking at work, nearly half of the
companies responding to our survey still do not have social networking policies.

Companies see the value in social networking sites as a way to promote their products and services without dishing out big bucks for traditional marketing. Most commonly, organizations set up corporate-branded accounts that are in some ways nameless entities on sites like Facebook and Twitter, said Nate Elliott, principle analyst for Forrester Research. But with so many of their employees already using these sites for personal use, he added, employers are looking to see how they can leverage their reach.

“It’s a really fine line,” says Elliott. “You don’t want to astro turf, create fake grass roots, and you don’t want to command your employees to use personal networks to promote the company.”

Indeed, employees need to tread lightly when they start using their contacts and friends from LinkedIn or Facebook for work purposes. If you leave your job, for example, it’s unclear if those connections belong to you or your employer, labor law experts say. Before the social networking explosion, if an employee had a list of contacts at work established for the sake of generating business or marketing, courts often decided workers had no right to those lists when they left their jobs.

“This is going to be interesting,” says Patricia Abril, assistant professor of business law, School of Business Administration at the University of Miami, about how laws will adapt to this latest Internet machination. “I think what’s at the core of this is a blurring of social and professional. Before it was a lot easier to establish when you were wearing your work hat and when you weren’t.”

There is little if any case law in the United States regarding this issue, but Abril pointed to a United Kingdom case that found in the employer’s favor. A longtime employee of Hays, a London recruiting firm, had been encouraged by his employer to use LinkedIn for his work, but when he left, his former managers accused him of stealing those contacts.

Although laws are different in the United States, Abril says the case is a precursor to the legal battles we may see playing out in American workplaces in the near future as more employees are asked to use social media as part of their jobs.

Beth Harte, marketing and communications guru, believes if someone puts in the time and the social capital to build up contacts, the contacts should be theirs. That said, she advises employees to negotiate upfront that their contacts are their property, especially if they are hired for a job because of their thousands of followers and friends on Twitter and Facebook.

Before you tweet or post a status update, you should know there are few laws to protect you if you say something dumb about your company on a social networking site and your boss fires you — even if it was your boss who encouraged you to go on Facebook in the first place.