FORTUNE -- Mobile applications that emphasize anonymous sharing are increasing in popularity, but two reign supreme: Whisper and Secret.
The former is the more established; the latter is the buzzy up-and-comer. But their appeal is essentially the same: Without revealing their identity, people can quickly post brief messages -- personal thoughts, secrets, questions -- to other people, who also remain anonymous (and can comment on or like the post). In practice, Whispers and Secrets range from the mild ("I can't stand rude people on the subway") to sweet ("I love my husband") to heartbreaking ("About two years ago I attempted suicide. No one knows.") to scandalous. ("I'm glad he was fired. Now he knows what it's like to be excluded.")
The commonalities between the two companies end there. Whisper, which has been around since mid-2012, has raised $54 million through several rounds of funding. ($21 million is from Sequoia Capital alone.) Secret launched only two months ago, but has become an overnight phenomenon, especially for the Silicon Valley tech community, whose members like to use it to gossip about their peers. Secret has raised $8.6 million from supporters like Garry Tan of Y-Combinator, MG Siegler of Google Ventures, Ashton Kutcher, and others. It has reportedly held discussions with Facebook to partner.
The functionality of the apps differs. Whisper is available on Apple's iOS and Google's Android; Secret is currently available only on iOS. Whisper allows all of its users to share with each other -- a veritable ocean of anonymized thoughts. Secret is more like a tributary to a larger river; it surfaces people (friends, friends of friends) in your feed based on the contact information stored in your phone. Secret's narrower approach gives you more relevant information, which may appeal ("I wonder who this is ...") or detract ("Ugh, I know exactly who that is ...") from the voyeuristic experience.
Another major difference: People can send anonymous direct messages using Whisper, but not using Secret. This has given Whisper a different dynamic; its users will ask each other for advice, chat for fun, arrange hook-ups, and more. (Secret users merely people-watch.) Whisper first charged users $6 per month to use messaging, but quickly turned the feature free. It now is working to experiment with advertising based on keywords found in a message: An update about "confidence" or "partying" or "fresh" might prompt a branded image of body spray to go with it.
Forrester Research analyst Julia Ask says there are a handful of revenue models for communication apps: You can pay to download an app; you can pay for features inside a free app; you can pay for features inside a paid app; you can collect on advertising revenue. The last one is the hardest: "You have to be able to target where the money is," Ask says. "Mobile is all about knowing who people are, where they are, and what they like. It doesn't work with anonymous apps."
Whisper's head of business development, Michael Downey, isn't letting anonymity get in the way of business. "Identity is the default, and that’s not going away," he tells Fortune. "Sometimes people want to do things that are not constrained by identity. We want this to be a medium that will be used by hundreds of millions of people."
Which is why Whisper's latest venture is to become a source of original editorial content. The company recently hired Neetzan Zimmerman, formerly of the gossip blog Gawker, to be its first editor-in-chief. Zimmerman's job is to work with external publishers to establish content partnerships that allow Whisper content to be featured outside its network. For example, Whisper and the news entertainment website BuzzFeed now have a partnership where, in exchange for BuzzFeed gaining access to Whisper's internal search tool and information about its network, Whisper posts will appear on BuzzFeed.com. "The content partnerships have no financial elements to them," Zimmerman says. "Our content is unique, and the likelihood of coming across similar content is very difficult because of the anonymity that we afford our users. So we provide content that creates strong engagement with their audience. In return, we establish brand presence."
Zimmerman says he'd like to make Whisper a platform for serious news, much like Twitter." One of the things we see come up again and again," he says, "is the nature of our users to divulge information about their surroundings when it’s something that they’d be worried about associating with their identity. Something might be happening in a country where you’re susceptible to punishment for speaking your mind. We see that people are turning to Whisper in an effort to get their message out."
Secret declined to comment on its business strategy for this article, but did say that it is in the process of hiring staffers to manage its rapid growth. The company announced in a March 14 blog post that "75 percent of people with more than five friends come back every day [and] 90 percent of users that engage in a conversation come back within the week, often several times per day." In time, Secret could very well enjoy a user base as large as the hype that surrounds it.
Both Whisper and Secret are banking on the notion that, in a world where it is increasingly difficult to use digital products without leaving a digital footprint, anonymous apps are more than a fad. "There is," Zimmerman says, "a level of comfort in anonymity."