“NO!” came one shrieking response via text; another friend shrugged, wondering why I was troubled about being unmasked. After all, I live online with few qualms. Facebook and Twitter, plus Foursquare, Tumblr and Instagram, are just the tip of the iceberg. Still, I wasn’t expecting fragments of my online persona to collide in such a jarring way. I’d left out specifics about myself, first to observe that dating site undetected, then to reinvent myself as an eligible bachelorette.  In the end, I didn’t go on the date. I wanted to introduce myself to a handsome stranger at my own pace, rather than be exposed in one fell swoop. I couldn’t reconcile the tectonic imbalance in power and information that came with the note: He knew so much about me, and I knew nothing about him.  But the experience raised a question I haven’t been able to shake. As digital identities become increasingly persistent across the Web, is it still possible to reinvent oneself online?  “We are all going through the uncomfortable experience of discovering just how much information about ourselves that we put out there,” said Ethan Zuckerman, a researcher at the Berkman Center for Internet and Society at Harvard, who studies online expression and the digital world. “As we casually go about our business, we are leaking all kinds of data that someone can piece back together.”  Such discomforting clashes between the lives we lead online and those we lead away from the keyboard are likely to grow in frequency. But there could be bigger issues here than wanting to keep a dating profile discreet, Mr. Zuckerman said, like struggling to protect the identity of a political whistle-blower or a victim of abuse. Retaining anonymity becomes more challenging as the Web populace becomes more interconnected.  “Staying under the radar is very hard to do while using the full features of the Web,” he said. The challenge, he said, is to understand how technology can coax users into sharing more than they might otherwise.  Companies that do business online increasingly tailor their sites to individual users. Netflix, Amazon and Pandora all note your preferences to make their services more useful. The same goes for Facebook and Google, which digest clicking and browsing behavior to customize links and information you see, based on the information you share.  “The future of the Web is to personalize,” said Amit Kapur, the chief executive of Gravity, a start-up in Santa Monica, Calif. “It’s driving a paradigm shift that will change the Web from theirs to ours to yours.”  Gravity, a tool that “mines your interests from Facebook and Twitter to present things from around the Web to you,” is tweaking its software, he said, with the aim of performing functions like delivering improved restaurant recommendations and powering personalized news readers. Such services help companies customize advertising for Web consumers.  That may be fine for businesses, but what do we lose when we can’t mutate and molt through online personas? There’s something deliciously freeing about shedding one’s self to don a shiny new identity. It’s why vast multiuser online games like World of Warcraft have flourished and why the anonymous video-chatting site Chatroulette catapulted in popularity.  The most common case against anonymity is that it gives rise to bad behavior online, allowing a docile Web denizen to slip from a Dr. Jekyll into a Mr. Hyde. And while this is a real problem, some advocates of Web anonymity, like Christopher Poole, the founder of an online community called 4chan, say people should be able to separate their online and offline identities.  “There is always a need,” he said, “to be able to enter into a conversation and have your contribution judged for its merit and not who you are.”  Mr. Poole and some other entrepreneurs are trying to build some layers of anonymity back into the Web. He says he’s doing that with a new company, Canvas Networks, that will experiment with an online community that will allow some identity concealment. Others are creating tools and carving out areas on the Web to preserve discretion. For example, a tool called Disconnect disables third-party tracking while Web surfing. And a search engine called DuckDuckGo does not collect browsing history or any personal identifiable information, its creators say.  B. J. Fogg, a psychologist at Stanford, suggests that in the future, people will not move about the Web undetected or swap identities as easily as a Halloween costume.  “People are not going to go back from disclosing everything and living out their lives online,” he said, adding that an evolutionary shift toward greater online openness is under way.  “The genie is out of the bottle,” he said.  Maybe Mr. Fogg is right, and the demands of a digital lifestyle have set a larger cultural transition into motion. But there is something of a covert resistance afoot, the fringes of which I can see on the Facebook page of my 13-year-old niece. She and her friends use only cute screen names to identify themselves, and the only profile pictures they post are rendered nearly unrecognizable by cartoon hearts and sparkles. Maybe it’s a start.  A version of this article appears in print on December 19, 2010, on Page BU3 of the New York edition with the headline: So Much For Reinventing Ourselves Online. Order Reprints| Today's Paper|Subscribe
When profiles were anonymous, users could better separate their online and offline lives. Now a presence on a dating site may lead to Foursquare, Twitter and Facebook.