Demonstrate you are worthy of the introduction.

After Sandy Baldonado applied for a number of promising positions via professional networking site LinkedIn and received no response, she began to wonder: was her resume being purposely or accidentally overlooked? Her marketing skills were a fit. But in some cases, Baldonado wouldn’t hear a peep only to see the same job posted again a short time later.

“Should I reapply or try and contact the hiring manager?” Baldonado wondered. So far, the San Francisco Bay Area-based branding and digital marketing specialist has taken a wait-and-see approach. “I don’t want to come across as too aggressive,” she said.

Baldonado is likely not alone in her mystification of how to navigate this new online world of job searching. The explosion in popularity of professional networking sites, such as US-based LinkedIn and Germany-based Xing, has brought with it a whole host of new do’s and don’ts when it comes to proper protocol for job seekers.

The reach of these sites is tremendous. LinkedIn has more than 238 million members in 200 countries and territories, while Xing has more than 13 million members mostly in Europe.

About 72% of companies in the US now use social media to advertise their jobs, according a recent survey of 350 hiring employers and 2,117 job seekers by Southern California-based CareerArc Group. One in three job seekers use social media as their primary tool for job searching, the survey showed.

Try, try again?

Reapplying for the job may not get you the interview you want — especially if it didn't work the first time around, according to Ashley Ringger, managing director of Switzerland-based Set Sails Social Media. But that doesn’t mean you shouldn’t do it.

“To increase your chances of getting picked from the bunch, make sure your LinkedIn public profile is complete and create an online resume to use as a supplement to your normal application packet,” Ringger said.

Put aside worries about appearing overly aggressive. “Don't be apprehensive in using these methods when applying for jobs,” said Ringger. “LinkedIn and other platforms are there for networking and connecting.”

Read the signs

Consider how active the hiring manager or recruiter is on social media to determine whether you should follow-up via LinkedIn or a personal email to the recruiter’s work account.

If the hiring manager has, say, more than 200 connections and has robust recent activity, it’s likely he or she regularly uses LinkedIn as a hiring source. In that case, sending an InMail (LinkedIn’s internal email system for members) could be a smart way to reach out. Keep your note short and to the point. Think about how you would want to be approached and then use that technique in your note, said Lisa Rangel, managing director of New York-based Chameleon Resumes LLC and a former finance and accounting recruiter.

But there’s an even better approach than sending an InMail: find a mutual LinkedIn connection to introduce you, suggested Karalyn Brown, founder of Australia-based job site InterviewIQ. “That way you are referred in via a trusted source,” she said.

Don’t just whip off a note asking the person to put you in touch; put some thought into what you send. Ask the connection for insight first: Does he or she know if the position is still open or whether it has been filled? If it’s still available, can he or she connect you with the hiring manager? Mention a skill that shows you could be a good fit for the job and consider including an unusual fact you’ve learned about the company.

“Make it clear you've done your homework and ensure your profile is completely filled out,” said Nicole Williams, LinkedIn’s career expert. “You need to make this as easy on them as possible and demonstrate you are worthy of the introduction, because ultimately it's going to affect their professional reputation.”

If the hiring manager with whom you want to connect appears to only use LinkedIn or other networking sites occasionally, then you’re probably better off sending an email to his or her corporate account.

Rangel recommends employing pleasant persistence. “It often takes five to six approaches to get a response, so do not give up before the response happens.”

Small dose interaction

Hiring managers really do want to talk to job applicants, according to InterviewIQ’s Brown. Of 25 hiring managers she interviewed for a book she was researching, 24 said that they welcomed contact from job applicants — but with stipulations.

“The person approaching them needs to state very specifically why they want to work for them and the value they can add,” said Brown. “Otherwise a contact is not much better than spam.”

Don’t assume the worst

Just because a position is reposted doesn’t mean that you weren’t a good candidate for the job. The position could be under review or the company may have hired someone and that person turned out not to be the right fit, according to LinkedIn’s Williams.

“More often than not, they simply haven't filled the position yet and your due diligence will put you on top of the pile,” she said. “And if the position has been filled, all the work you've done and connections you've made will lead you to a new opportunity.”    

Career Coach is a twice-monthly column on BBC Capital in which we consider the career turning points and questions many professionals face. We welcome questions from readers at careercoach@bbc.com.