The answer could be as simple as location. Although the implementation could be complex, the basic premise is that although mobile phones do not have cookies, they do have location. Smartphones get taken from place to place with the user, logging in to various different networks -- and with the owner's permission, denoting GPS coordinates. So could location be a way of tracking a user and thereby, finding out more about them?
It's probably a moot point for the likes of Google, Apple and Facebook, which will have log-in data gleaned from the user's smartphone, as do games developers, to a lesser extent, when users are logged into their app to have a go at "Angry Birds." For the rest of the advertising industry, building up an image of a person is always more complex.
Thus, at the end of an intriguing panel at MediaPost's OMMA Audience Targeting event in London on Monday, representatives from Addictive Mobile, MediaCom, Millennial Media and xAd agreed that a very simple premise ---location -- is mobile's cookie.
If a user frequents a gym throughout the week, you can be pretty sure that is a strong sign they take fitness seriously. If they are in and out of coffee shops or a bar, there is a useful sign there too of the types of real life activity they are interested in.
There is also the area of probability. Tools are being widely used among agencies to reveal who someone is (in terms of interests rather than their actual identity) based on what they have done online and what they appear to be doing on their mobile devices. The claim was made at Monday's event that this can be accurate in around three in four or four in five cases -- and so, although it is not good enough to be relied on, gives a pretty good indication.
The proverbial elephant in the room, other than the simple conclusion that location is the key to understanding users, was that now the digital world has tipped in favour of mobile, and will continue to do so, understanding mobile users' habits is becoming as important or even more important than desktop learnings.
Thus, although the argument today is always how we take online anonymised customer data and apply it to mobile, the future will see the question reversed. It will witness marketers asking how they take the lessons of mobile use and apply them to online because the real gems are more likely to come from mobile engagement and brand interactions.
So for those looking at the cross-device issue or simply pondering how to target mobile users better, the answer from Monday's event was very clear.
Location really is mobile's cookie.