LIKE many 29-year-old Web designers from Brooklyn, Kelsey Rahn spent part of New Year’s Day hung over, poring over a list of resolutions. “Make more time for myself.” “Make soup in my Crock-Pot twice a month.” “Join the gym by Friday.”

The only thing was, she wrote that list by hand one year ago. It had resurfaced in her in-box — in the form of a Twitter-linked photo she had taken of her handwritten note — thanks to Timehop, a new service that turns subscribers’ social networking activity into a kind of personalized time capsule.

“It was fun to see what I was thinking at this exact same time last year,” Ms. Rahn said.

The site, which began as several distinct sites, including 4squareand7yearsago, became Timehop last month. It sends users a daily e-mail reminder of what they did a year ago, and it does so by retracing the subscriber’s digital footsteps on the social media sites Facebook, Twitter, Instagram and Foursquare.

When so much of the iPhone generation seems to tweet for the moment, the service, which is free, metes out a daily dose of introspection. The concept has already attracted tens of thousands of subscribers, the company said.

Nicole Thomas, 23, an art director from Brooklyn, said she nearly cried when Timehop reminded her of her good fortune in landing a job last year. Austin Sevener, 26, a survey programmer in Fort Worth, said he hadn’t realized that he had bought his wedding ring on the same day as a lunar eclipse.

It is “the same instinct that drives people to write diaries,” said Jonathan Wegener, 26, a Web entrepreneur in Brooklyn, who started the site with Benny Wong, also 26, a programmer. “That desire to sort of capture a moment in time and capture your thinking and your world so that you can look back on it later.”

Unlike Facebook’s controversial new timeline feature, which makes a user’s Facebook history more readily viewable by friends (including those embarrassing old college photos), Timehop is more private, since the e-mails are sent only to the subscriber.

Aram Sinnreich, an assistant professor of media studies at Rutgers University in New Brunswick, N.J., said that Timehop is part of “a radically new moment in our cultural history” emerging around what he called “the quantified self.” A new wave of Web applications, he said, is empowering people to track, sort and analyze their own online behavior — in much the same way that Web sites and advertisers already track and analyze consumers.

It’s also changing how we view ourselves. “We are beginning to see ourselves not just from the inside, as an actor doing something on a daily basis, but from the outside — understanding what we look like to the world around us and developing a kind of hybrid identity,” Dr. Sinnreich said.

David Fraga, a business developer for Shutterstock, a photo agency in New York, said that using Timehop has made him more conscious of the future memories he is creating on social media sites like Facebook. “I’m not just sharing with friends but also with my future self,” he said.