It’s difficult to talk about consumer-facing web services without talking about online identity.

What is online identity? Ev Williams laid out five ways he thinks about identity online: as authentication, representation, personalization, communication, and reputation. Caterina Fake argued anonymity and pseudonymity are important to identity in social software. Rebekah Cox posited mobile identity is about the distribution of attention, a fixed resource. There’s a thriving “Online Identity” topic on Quora with other good stuff.

In short: there’s no strong consensus about what “online identity” is or is not, but there’s no strong consensus about what “offline identity” or quite simply “identity” is or is not, either. The web, unsurprisingly, isn’t much different from everything else.

There are web-specific pieces to the identity question though. Take authentication, which is used because online interactions don’t (yet) have the certainty of face-to-face interactions. Discussions of authentication tend to assume there will be one source of online identity — call it a global, centralized White Pages. The scent of gold rush surrounds these conversations: it’s assumed that the owner of the White Pages will be able extract significant economics from their prize.

Authentication schemes set up their own questions like “Should I use Facebook/Google/Twitter/OpenID/etc./etc. as my primary/secondary/only method of authenticating new users?” Those questions are rabbit holes, and they obscure an important distinction between identity and its interface.

Identity is who you are — it’s all those things that Ev laid out, and potentially others, and it’s likely got characteristics that Caterina and Rebekah mentioned.

On the web, because we’re not speaking face-to-face, we’ve got to access that identity somehow. How we access that identity is it’s interface. Here’s a few examples:

Identity Interface
Phone number Phone book entry
Facebook ID Facebook user name –> First name Last name
Twitter ID @twitterhandle –> First Name Last Name in following list
Google+ ID +Google+ name –> Name in Google Contacts
Tumblr ID Tumblr username
Foursquare ID First Name Last initial
Yelp ID First Name Last initial
Email address Name in Google Contacts/Outlook address book/Mail.app address book

There’s definitely deeper infrastructure built around the concept of “identity” in each service beyond a user ID. Beyond a user ID is who people are, what they can do, how they do it, and with whom they do it. A robust concept of “identity” can answer all of those questions, and it’s at the core of why people use the services and tools they do.

The interface to identity in a web service is its topmost layer: it’s what users see, so it’s sticky but not irrevocably so. You see slow changes, like Google+’s moves to +Name rather than @Name or @username. (They’re accepting both syntaxes in the short term, hoping they can train users while minimizing frustration, I’d imagine.) You see Facebook adopt and then move away from usernames toward First Name Last Name. You see Tumblr hang on to its usernames that allow for anonymity, pseudonymity, and multiple personas.

Each service has its own concept of identity that’s distinct from what that identity initially looks like or feels like when it’s used. Controlling what an identity means is likely very powerful whether or not you control what it looks like.