FORTUNE — Dear Annie: The last time I looked for a new job, about four years ago, the most discouraging part of the process was applying for a position, even going through more than one interview, and then hearing nothing back. Now, it’s happening again. I applied for an opening at a company where I’ve always wanted to work. They called me in for an interview, which I think went really well, about three weeks ago. I’ve followed up by phone and email a few times to reiterate my interest since then, but I’ve heard nothing. Nada. Not a peep.

Meanwhile, another company has offered me a job that I guess would be okay — better than where I am now, anyway — and I don’t know what to do. I could accept this offer, but then what if the company I’d really prefer finally gets back to me? How long should I wait before assuming I didn’t get that job? — In the Dark

Dear I.D.: Maddening, isn’t it? I hear this question constantly, sometimes even from people who have flown clear across the country for a round of interviews and then have heard…nothing — not even an email that would take 20 seconds to send, saying for instance, “Thank you for meeting with us. The job has been filled, but we will keep you in mind for future openings,” or words to that effect.

Absolute silence is rude, inconsiderate, and makes people mad. “It’s human nature to expect some kind of response,” says Chris Forman, CEO of an application-tracking site called StartWire. “And when candidates feel an application has vanished into a black hole, especially if they’ve put considerable effort into it, they get p.o.’ed.”

Demoralizing as it is for job hunters, leaving people hanging is bad for companies too. “What HR people and hiring managers are just starting to realize is that neglecting to let candidates know where they stand is damaging their companies’ reputations and their brands,” Forman says. A new StartWire survey found that 77% of jobseekers think less of a company that leaves them in the dark, and more than half would decline to buy or recommend that company’s product or service.

Moreover, the Internet exponentially increased disgruntled candidates’ ability to spread the bad word. “Before the Internet, if a company treated you shabbily, you’d tell maybe 10 people about it,” says Forman. “Now, you can post your experience on sites like Glassdoor.com, Vault.com, and Facebook, and tweet all your followers. A negative experience can quickly go viral.”

He adds that a typical big company starts with an average of about 30 applications for each opening it fills, “so if you hire 1,000 people a year, you’re interacting, for better or worse, with roughly 30,000 candidates. And alienating 30,000 potential customers, plus all their online contacts, is not very smart.”

The irony is that it doesn’t have to be this way, again because of the Internet. Over the past five years or so, most large employers have adopted sophisticated web-based recruiting tools, which have built-in features that keep track of the status of each candidate’s application.

“These features work the same way as when, for example, you order a book from Amazon (AMZN). The system tracks when it leaves the warehouse, when it goes on a truck, when it’s en route for delivery to you, and so on,” says Forman. “So the problem isn’t that employers don’t know where your application stands, how many other people are in the running, or whether the job has been filled. They know precisely.”

They just don’t tell you. StartWire’s research shows that only 33% of Fortune 500 companies pass along any of the data they have on hand to candidates, even though 90% of job seekers surveyed said that getting that feedback would make their job hunt “less frustrating,” and 96% said they would be more likely to apply for a job at a company where they know they’ll be kept informed. “Companies that are notorious for ‘application black holes’ lose out on potential star employees,” Forman says.

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With all that in mind, how long should you wait before concluding that you didn’t get that job you really want? As a general rule of thumb, a job stays open for about 45 days. “So if you know when the opening was first posted” — one of the many kinds of data that StartWire collects from about 5,600 employers and makes available to its users — “count 45 days from that date, and if you’ve still heard nothing, you can assume you didn’t get it,” Forman says.

In your case, since it’s been three weeks since your interview, it’s still about three weeks too early to give up. But is the employer who did make you an offer willing to wait that long for your decision?

Probably not, so at this point, “you should contact the company where you’d prefer to get hired and let them know you have another offer,” advises Annie Stevens, a managing partner at Boston-based executive coaching firm ClearRock. “Frame this as a courtesy to them, and invite them to make a counteroffer.”

What if you do that and still hear nothing? “If you don’t receive a counteroffer within two days,” says Stevens, “then take the other job and make the best of that opportunity.”

Good luck.

Talkback: Have you applied for a job (or more than one) and been left in the dark as to whether you’re still in the running for it? If you’re a hiring manager, do you agree that applicants should wait 45 days before giving up? Leave a comment below.