But you can choose to stay hidden if you wish

A new Yahoo service for connected mobiles can tell you when friends are nearby.

The service, one of many on show at Mobile World Congress (WMC), that use the location facilities of GPS or cellular triangulation, could mean an end to those occasions when you miss friends who visit the same places at the same time.

But, unless you are careful, it could also mean you being hassled by a lot of people you would rather miss.

Marco Boerries, executive vice-president of Yahoo! Connected Life, said the company had thought a lot about the privacy issue.

“We made it so that you are opted out by default,” he said. “You have to choose to be visible on the system.”

You can also choose what spectrum of acquaintance the system can cover: family, people in your contacts book, or people who have you in their contacts book.

Boerries gave a demonstration in which the five locations of five of his friends (or contacts, as they say WMC) on a little map of the fairground area. By changing the resolution you can locate people over a wider area.

The facility is part of a package of service called Oneconnect which will provide access to different social networking sites, IM networks, and email systems. It is broswer based, and so should work on any platform.

Yahoo! is beating Google on mobile searching, which may be one reason Microsoft offered $44.6bn for the company. The Yahoo! Board has rejected the offer.