FORTUNE—I can’t figure out what Weng Ian is doing. The two of us are guests at a bridal brunch in Tupelo, Mississippi, standing in the formal living room of a home in which dozens of women in festive attire are packed elbow-to-elbow. Some are chatting politely in pairs, sipping mimosas between introductions. Others are fawning over the bride-to-be, peppering her with questions about the day of the wedding. Smells of cheese grits and bacon waft from the kitchen, where volunteers are preparing a decidedly Southern morning meal.
No one has a cell phone out. Then again, no one is 17.
Except Ian. Beside me, she is holding up her phone, contorting her face and giggling to herself as she rapidly taps the glass display. Ian is a high school senior with a mane of long dark hair, gold dangling earrings that frame evenly set features, and her heart set on attending Vanderbilt next year. She explains to me that if this were her party, everyone would be clutching a phone. “You know,” she says. “Sending snaps."
Anyone who is not a teenager (or a tech reporter) can be forgiven for not knowing about Snapchat. It’s this season’s hit photo-sharing application—its signature feature is that users can only see a photo for up to 10 seconds, and then it disappears forever. The service has seen user adoption jump 163% since January, according to Comscore, to 10.9 million people posting photos on the service. (It’s worth noting that Comscore only measures American smartphone users over 18; most of Snapchat fans are, like Ian, younger teens.) Spiegel says Snapchat users send 400 million photos every day.
With action like that, everybody wants in on it. Facebook is said to have recently offered founders Evan Spiegel and Bobby Murphy three billion dollars for it. Press reports suggest the company is raising a round of funding that may value it at more than $3.5 billion. There are plenty of credible people—journalists, analysts, even venture capitalists—asking whether this company could be worth so much money.
I’d like to ask a different question: Why is Snapchat so popular?
Because our relationship to photographs is changing.
We once relied on photos to document events. Pictures were (physical, then digital) objects that we tried to collect, preserve and share. Photographs captured the world around us, with increasing frequency as technology made them easier to obtain.
Today, we carry smartphones on our bodies that offer the uninterrupted ability to take a photo and share it, thanks to myriad “social” services. In response to this pervasiveness, photographs have become something new: a way to communicate, visually, and in real time.
Back at the brunch in Tupelo, Ian taps a yellow icon on her iPhone screen, and her camera software appears, its lens pointed at her and not the bridal festivities beyond. She pushes up the tip of her nose, wrinkles her eyes, and sticks out her tongue, then snaps a photo of herself—a “selfie.” It’s blurry and unflattering. With a flick of her left thumb, she sets a timer on the photo for three seconds, and sends it out to a half-dozen of her friends.
While we talk, Ian’s phone chimes as friends return her “snaps” with similarly unbecoming photos. With a digital fluency that allows her not to lose the thread of our conversation, she creates and responds to snaps as we dig into our meal. I ask her what the point of the exercise is. “It’s like, we take bad pictures of ourselves and send them back and forth to each other, maybe a dozen,” she tells me.
If the photos were words, I ask her, what would the words be? “It’s like, ‘Hey, what’s up?’ Or, ‘I really hate this class, get me out of here,’” she replies. “It’s like a picture conversation.” No words necessary.
With the rise of Snapchat and services like it, it’s clear that the reign of the written word as the exclusive medium for real time communication is coming to an end.
Inherently, we know this. We text a grandparent photos of the kids falling asleep to suggest that all is well at home. We send a picture of an empty carton of milk to a spouse at the grocery store. We “insta” a photo from an early morning walk to share that invigorating feeling with a small circle of followers.
We also turn to photos to communicate with broader, more public audiences. I was in Istanbul’s Tahrir Square when the government attacked protesters in May. Protestors took to their cameras, transmitting images through Facebook and Twitter to communicate their concerns to broader global audiences.
In all of these examples, the primary purpose of the photo is not documentation, but communication. Like a phone call, these photos aren’t intended to be stored so much as absorbed, decoded, and released. They are temporary, intended to foremost convey an immediate message.
Services that imitate or interpret Snapchat’s ephemeral approach are popping up everywhere. Twitter has introduced a new design that puts photos at the forefront. The following of Facebook-owned Instagram continues to expand. And dozens of photo-sharing messaging apps like MessageMe and WhatsApp and the Tencent-owned Chinese app Weixin are emerging. (A recent personal favorite is FrontBack, an app that lets people capture and share a photo from the front of the camera paired with a photo of what’s behind the camera, usually a selfie.)
Which goes to say that none of the existing technology giants dominates photo messaging yet. This is the reason Facebook is willing to pay billions of dollars for Snapchat, and why its founders and investors are confident enough to reject it.
But it’s a dangerous play. In decades past, communications platforms worked because everyone was on them and because there was material cost to switching. The original landline telephone, for example, was expensive and wouldn’t work as a reliable means for communicating with others until most of the people you sought to contact had one. Today, there is extremely little cost to switching: it takes less than five minutes to download a free photo-sharing app, see who you know is using it, and connect to your most frequent contacts on it.
It’s likely that Ian and her friends will use Snapchat until they tire of the app. And then they will bring their pictoral conversation someplace else.