Before you post that picture of you and your friends hanging out with those strippers in Vegas, remember that your future employer might be watching.

Since the advent of Facebook, people have had to worry about how the new culture of completely open information could affect their reputations. Some worry more than others – kids with strict parents, college grads trying to get into professional school, people in the public eye, for instance. But to a certain extent, we should all be wary of what we post on social networking sites like Facebook, LinkedIn, and Twitter (or at least learn how to use privacy settings). That’s because a significant number of employers are still patrolling those sites for reasons not to hire your debauchery-loving ass.

In fact, in a study conducted by Career Builder, 37% of hiring managers and human resource people from various industries said that their online background checks for prospective employees includes social media. That number is actually down from last year’s similar study (50%) and also less than 2009 (45%) – but it’s still nearly two in every five employers.

It could have something to do with the fact that 15% of those surveyed said their company has specific rules barring the practice.

The survey also unearthed 11% who say that they plan on scanning their employees’ Facebook profiles in the near future.

Out of those 37% of HR professionals and hiring consultants perusing through your social media accounts, one-third of them say they’ve found reason there to not hire a job candidate. The top reason? 49% say that inappropriate photos/info threw up the red light. That was followed very closely by “info about drinking or using drugs,” at 45%.

Other red flags: “Poor communication skills,” which I can only assume means so sound like an idiot in your statuses or tweets. Also, talking bad about previous employers is a no-no (33%) said that they have nixed a prospective employee because of that. Nobody wants an employee that’s going to publicly trash their employer if things go south.

Of course, it’s only mostly the bad stuff that people care about. There’s a chance that something you do or say on a social media site can actually help your chances in landing a job. According to the study, 29% said that they have hired someone because something they saw on Facebook or Twitter boosted their opinion of them as a person.

The most cited ways that social media made a good impression include showing a good personality, conveying a professional image, showing a well-rounded candidate, displaying creativity, and reinforcing qualifications.

If some of this comes out sounding like mixed signals, that’s because there are many fine lines here. In terms of landing a job with these social media hounds, you have to have a well-rounded personality, but not be too into the booze. You have to be creative but also professional.

The best advice is to use it to your advantage. Just cut out the bad stuff, and overplay the good stuff:

“Job seekers should be mindful of what potential employers can learn about them online,” says Rosemary Haefner, vice president of human resources at CareerBuilder. “If you choose to leave social media content public, tailor the message to your advantage. Filter out anything that can tarnish your professional reputation and post communications, links and photos that portray you in the best possible light.”

Finally, if you want to be a little edgy with your social media presence – it looks like Twitter is the place to do it. While 65% of the hirers said they look at Facebook and 63% said they look at LinkedIn, only 16% use Twitter in their social media background checks.

Of course, all of this only takes into account social media investigations from the outside. If your future employer wants to take a look at your accounts from the inside, you might have a whole other problem altogether.