The idea is pretty simple. By gathering all of this information, Exterion Media can show advertisers the types of people that make particular journeys. Knowing that large swathes of fashion-conscious Instagram lovers stand on the platform near Covent Garden, for instance, could inspire ASOS or Urban Outfitters to take out an ad. Similarly, knowing when certain users descend on a station -- Stratford is normally a place for shoppers, but the Olympic Park occasionally attracts sports fans -- could help companies figure out when to debut a highly targeted poster.

O2 has 25 million customers and expects to track roughly one million journeys each day. If you're worried about your privacy, there's an obvious solution -- switch off your phone's Wi-Fi connection and do something else while you're sitting on the Tube. Sure, you'll have to wait a little longer to check Twitter or Facebook, but there's always an alternative, like listening to some podcasts or ploughing through your Pocket queue.