Facebook unveils its latest trick – but the implications are worrying

I've just used my phone to inform friends that I've been standing at Tooting Broadway underground station. I'm not sure why. You could interpret it as narcissism – although, let's face it, we're only talking Tooting Broadway here – or self-obsession, or just plain neediness.

But the main reason I've done it is because I can. Facebook Places, a location-sharing feature within the world's pre-eminent social networking service, was enabled in the UK yesterday about a month after its US launch. Thanks to GPS technology, Facebook members can now track their friends' movements (should those friends want them to know), broadcast their own to-ings and fro-ings, and even see the names and photos of strangers who happen to be "checked in" at the same location.

Location sharing has been long predicted as the next big thing, but too many ambitious services have been chasing too few users who have shown only mild interest.

The current market leader, FourSquare, has some three million users worldwide, but that's small fry compared with Facebook's half a billion sign-ups. So why bother to join a stand-alone location service, and persuade your friends to join it too, when Facebook now does it all out of the box? Indeed, a recent survey of FourSquare users indicated that 82 per cent of them would jump ship to Facebook as soon as Places launched.

But Facebook's sheer size means it could face another outcry over privacy – not least because of misinformation that's been spreading over the extent of its capabilities. It's certainly not the case that anyone can see where you are at any time; you don't automatically appear as a pin in a map for the world to see.

If you're a Facebook member, but never use it on your phone, you won't encounter Places. Even if you do use Facebook on the move, you won't need to worry unless you press the "check in" button. But if you do start using it to tell people where you are, you'll need to be aware of who exactly can see what – as with many of Facebook's features. You may choose to share your location only with friends, but if you allow one of those friends to "check you in" on your behalf when you're out socialising together (a feature Facebook calls "tagging") that information will be seen by their friends, too. And while "friends of friends" seems a benign enough concept, on Facebook that can easily stretch to 10,000 people or more.

As ever, the onus is on the user to engage with Facebook's privacy settings – but, more importantly, to realise the possible consequences of location sharing. For example, a trivial white lie about where you've spent an evening could be unwittingly exposed to the world via Places; Facebook does allow you to remove any check-ins and erase tell-tale bits of information, but you could well realise too late. And the ability to see who is "Here Now" at a venue you've checked in at means strangers could immediately put a name to a face, and either approach you or send you a message – something that women in particular are likely to be very wary of. Then there's the obvious point that by checking in, you're telling people you're not at home. If they know your address, that's an open invitation to burglars; New Hampshire police smashed a burglary ring a few days ago that had targeted people precisely in this way, and recovered more than $100,000 (£64,000) of property.

So, what's in it for Facebook? Initially, a lot of flak over privacy. Longer term, a valuable stash of data revealing our patronage of local businesses which can then be used by advertisers. And for us? The chance to meet friends when we unexpectedly find them in the vicinity; the chance to meet new people and discover new places. And, of course, the opportunity to develop an aptitude for covering our tracks when "over-sharing" starts to get out of hand.

Q&A: Be sure to check out the 'check in' button...

What is Facebook Places?

A new feature introduced yesterday by Facebook, which will allow users of its mobile phone app (and mobile site at touch.facebook.com) to automatically share their whereabouts with friends using their phone's GPS technology.

How do I use it?

Choosing "Places" on Facebook's mobile app or site allows you to "check in" from a list of places nearby. When you do so, that information is posted on Facebook; you'll also be able to see a list of people currently checked in at your location. Friends can also "check you in", although you need to approve this each time it happens.

Who will know where I am?

The default setting means that only your Facebook friends know where you are. However, if your friends check you in, that information will be visible to their friends, too. Strangers checked in at the same venue will also be able to see your name and photo unless you specifically disable that function.

What about my children?

If "minors" (defined as 13- to 17-year-olds) choose to use Places, their location data will only ever be seen by people who are specifically listed by them as a friend on Facebook.

How can I change my privacy settings?

On the main Facebook site, click "Account" in the top-right corner of the page, then click "Privacy Settings", then "Customise Settings". You are then presented with three Places-related options: "Places I check into", "People here now" and "Friends can check me into Places". But the best way to ensure that your location remains private is, of course, not to use the service at all.