Facebook appears to have quietly unveiled a new feature designed to let people see which Facebook users are nearby at any given time.

The company calls the feature “Find Friends Nearby,” and on Monday it was available through Facebook’s mobile apps and website despite the fact that it hasn’t been formally announced. The blog ReadWriteWeb already has dubbed the feature a “stalking app” because it could open people up to potentially awkward or threatening interactions with strangers on the social network who know you’re nearby.

To test out the feature, go to fb.com/ffn in a browser, or follow this path in the Facebook’s mobile apps: menu > apps > find friends > other tools > Find Friends Nearby.

It appears that Facebook users must opt in to the feature by going to that site, otherwise their profiles will not appear in a list of people who are nearby.

The blog TechCrunch first reported on the feature Sunday after a non-Facebook developer tipped off the site to the feature’s existence. In a comment on that site’s post about the new feature, Facebook developer Ryan Patterson, who says he developed Find Friends Nearby, described how he hopes the app will be used:

“For me, the ideal use case for this product is the one where when you’re out with a group of people whom you’ve recently met and want to stay in contact with. Facebook search might be effective, or sharing your vanity addresses or business cards, but this tool provides a really easy way to exchange contact information with multiple people with minimal friction.”

A Facebook spokeswoman declined to provide additional information.

“We are constantly testing new features but have nothing more to share at this time,” she wrote in an e-mail to CNN.

Reactions to the news were mixed in the tech blogosphere.

The site Gizmodo finds the new feature to be potentially useful — and dangerous.

“You meet a cute somebody at a concert, you’re like, ‘Hey, we should be Facebook friends,’ and they’re like ‘OMG, totes!’ Then rather than having to spell your names and search around, you’ve got a much smaller group of people to choose from. I guess that’s great, right?” Brent Rose writes on that tech news site.

But Rose adds: “Sure, it’d be great to easily add a contact quicker, but imagine this: some creeper has been molesting you with his/her eyes all night. She opens the app, can kinda recognize your face from your profile picture, and now said creeper knows (your) name and possibly some of your personal info.”

That would be the downside.

Others say the app simply isn’t ready for prime time.

The blog Engadget calls the app “fairly primitive,” and The Next Web says it amounts to “nothing more than a parlor trick at this point” since other location-based friend finders haven’t caught on with the general public despite their buzziness in tech circles.

Facebook recently acquired a company called Glancee, which did something similar to the Find Friends Nearby feature. So there’s speculation that the company’s technology may have contributed to the new Facebook feature. Patterson, however, the Facebook developer, wrote on TechCrunch’s post that he created the feature during a hackathon.

Other similar mobile “social discovery” apps include Highlight, Banjo and Sonar. Some of these apps will also show you nearby friends of friends or even strangers who share your interests, based on your social-network profile.

It’s unclear exactly how much information the Find Friends Nearby feature gives away. For now at least, users have to log into that site intentionally to see a list of people who are nearby, and that list of people appears to be quite limited for now.

Also to be determined is how large of a radius the feature employs. Does it search for people who are within eyeshot of you? Or within a mile?

If you test the feature, let us know what you think in the comments section. So far, we haven’t been able to interact with anyone — friendly or otherwise — who’s nearby.