Dr Vassilis Kostakos, research associate at the University of Bath, said: "Networks are everywhere - social and digital.

"The really nice thing about Bluetooth is that when you are walking down the street, although you are not talking to anyone, your Bluetooth device can be talking to other devices.

"People with Bluetooth devices are actually creating an ad hoc communications infrastructure where information can flow through the city over time."

He said the project's motivation was in helping people find out more information about "familiar strangers".

'Publicly online'

He said: "Most people you bump into or see regularly have made information about themselves available publicly online.

"But the internet is such a big place that it's difficult to find contextual information about who someone is, where they are etc."

The tool lets users find out if any of the people they bump into regularly is a Cityware user and has a profile of Facebook. If so, they can then choose to add that person to their friends' list.

The tool works in four parts: Facebook account, Cityware application, Bluetooth device and Cityware node.

Users must have a Facebook account, install the Cityware application and register the Bluetooth ID of their mobile phone or laptop with the software.

The researchers have set up a series of nodes around the UK and at locations in the US.

These nodes are computers which constantly scan for Bluetooth-enabled devices in a given area, and send that information back to servers which compare the IDs of the gadgets with any enabled Facebook profiles.

Dr Kostakos said: "The node sits in the environment and records everyone's unique Bluetooth ID. Cityware itself doesn't know your name, or who you are."

Nodes have been set up in Bath, University College London, the University of California in San Diego, with more nodes going online in Sweden, Hong Kong and Sydney.

He added: "When you return to Facebook you will see a list of all the devices you were near and the link to profiles of people who have tagged themselves on Cityware."

The Facebook tool is part of a wider project looking at issues around pervasive computing - connected devices in the environment.

"We are interested in understanding how cities work, how people move around. More recently we have been looking at how viruses spread in cities - biological and digital viruses."

Reinforcing theories

He said analysis of encounters was reinforcing theories that viruses are spread through encounters that "last a long time and are frequent".

"It is also demonstrating that encounters that are short and infrequent help spread innovation and new ideas," he said.

The ambition for the Facebook tool is to have mobile phones alert each other when in the proximity of another Facebook user who shares common interests or common friends.

Dr Kostakos said: "This won't happen in the near future because it is very difficult to write an application that runs on a lot of phones. But it is possible."

The team is also working on an application for virtual world Second Life.