With digital communication tools everywhere, there has never been more data available to marketers about users. It seems like we shouldn’t have a problem building rich profiles of user behaviors, interests and motivators. But the chance to create deep personalization all hinges on a single question that brands must ask for each point of data: Who are you? Linking behavior to identity can be extremely tricky online, with users using multiple devices and operating under any number of user names, accounts and aliases.

Identifiers
Finding an identifier for an individual used to be as simple as reaching for the White Pages. Once you had a name and a town, there was little confusion about who you were dealing with. Now there requires a variety of efforts of figuring out who is who, including home address, social media handles, cookies, accounts, 3rd party tagging services and the standard social security number. Of all the ways to pin down an id, cell phone and email have become the most useful.

Limits of cookies
The standard way of tracking behavior for sites, user cookies, is still effective. But it falls short in an increasingly multi-device world. Since cookies are stored on the user’s local machine, your site may not recognize them when they return from their tablet. And cookies have always had built in challenges from multiple users using the same machine in a public setting or at home. Have you ever seen ads show up while browsing on the home computer that were more appropriate for your spouse than for you?

Cell phones and email
Phone numbers and email addresses are more of a true identifier since they are permanent for most people. Both are seldom switched in real life and actually facilitate contact. If you want someone to be able to actually reach you, you’ve got to make your phone number known. Relying on any email is problematic, since it’s easy for users to have multiple email accounts including disposable or ‘junk’ addresses. So for the purposes of evaluating the value of data points, I’m referring to actual personal email addresses that receive expected mail.
The cell phone number is probably the ideal unique key for keeping track of users in theory. With the ability to port numbers to any new phone, most consumers are loathe to switch their number for any reason and numbers are not quite as easy to fake.

Email for the win
The issue that tips the scales in favor of email is collection. Users aren’t as likely to share their phone number with brands and if they do, they don’t expect to receive marketing messages by call or by text. There are so many more opportunities to collect and use emails. Email addresses are readily shared, especially in return for valuable content. If they do sign up for a newsletter, brands can append their hyperlinks with a unique userid in case customers are consuming emails on a different device. They are also the standard username for social sites, which helps sites with social log-in features to link together separate visits to the same user.

The best choice of all is to make use of all identification opportunities and utilize them to tailor experiences as much as possible. But all data points available to collect are not created equal. Email will serve as the best foundation for building a robust personalized experience for users.