Twitter’s planned shortening of all links via its t.co service is about to happen. The initial motivation was security, according to Twitter:

“Twitter’s link service at http://t.co is used to better protect users from malicious sites that engage in spreading malware, phishing attacks, and other harmful activity. A link converted by Twitter’s link service is checked against a list of potentially dangerous sites. When there’s a match, users can be warned before they continue.”

Declan McCullagh reports that Twitter announced in an email message that when someone click “on these links from Twitter.com or a Twitter application, Twitter will log that click.” Such information is extremely valuable. Give Twitter’s tens of millions of active users, just knowing how often certain URLs are clicked by people indicates what entities and topics are of interest at the moment.

“Our link service will also be used to measure information like how many times a link has been clicked. Eventually, this information will become an important quality signal for our Resonance algorithm—the way we determine if a Tweet is relevant and interesting.”

Associating the clicks with a user, IP address, location or device can yield even more information — like what you are interested in right now. Moreover, Twitter now has a way to associate arbitrary annotation metadata with each tweet. Analyzing all of this data can identify, for example, communities of users with common interests and the influential members within them.

Note that Twitter has not said it will do this or even that it will record and keep any user-identifiable information along with the clicks. They might just log the aggregate number of clicks in a window of time. But going the next step and capturing the additional information would be, in my mind, irresistible, even if there was no immediate plan to use it.

Search engines like Google already link clicks to users and IP addresses and use the information to improve their ranking algorithms and probably in many other ways. But what is troubling is the seemingly inexorable erosion of our online privacy. There will be no way to opt out of having your link wrapped by the t.co service and no announced way to opt out of having your clicks logged.