If your in-box is currently reporting unread messages in the hundreds or thousands, you might have a hard time believing the news: e-mail is on the decline.

The total volume of e-mail has dropped about 10 percent since 2010.

The word “e-mail” itself tells you about its origins: it was modeled on written letters. To this day, a lot of e-mail begins with a salutation. Maybe it's “Hey” instead of “Dear Casey,” but it's there.

And because there was so much overhead involved with sending a letter—folding, enveloping, addressing, stamping, mailing—few bothered to send only a few pithy words. The effort seemed to justify a longer message.

Once heralded as the death of the personal human touch, e-mail has now taken over the letter's place as a ubiquitous form of communication—both business and personal. But is its day in the digital sun coming to a close?

At first blush, that might seem to be the case. The incoming generation, after all, doesn't do e-mail. Oh, they might have an account. They use it only as we would a fax machine: as a means to communicate with old-school folks like their parents or to fulfill the sign-up requirements of Web sites. They rarely check it, though. As of just a few years ago, among 25- to 35-year-olds, e-mail had already dropped 18 percent—and among teenagers, it was down about 60 percent. A Microsoft human resources guy recently told me that lately, applications from recent college graduates leave the “e-mail address” field blank. It's considered too unwieldy, uncool, not immediate enough.

Today's instant electronic memos—such as texting and Facebook and Twitter messages—are more direct, more concentrated, more efficient. They dispense with the salutation and the signoff; we already know the “to” and “from.”

Many corporations are moving to messaging networks for exactly that reason: more signal, less noise. And less time.

This trend is further evidence that store-and-forward systems such as e-mail and voicemail are outdated. Instead of my leaving you a lengthy message that you pick up later, I can now send you an unobtrusive, easily consumed message that you can read—and respond to—on the go.

The decline of e-mail corresponds neatly to the dawn of the mobile era. Let's face it: e-mail has historically been an activity. You sit down to do it. You fill up a block of time. And long missives are clumsy on a phone.

But instantaneous written messages are different. These are neatly tailored to fit in just about any time: before a movie, in a taxi, waiting for lunch. And because these notes are invariably brief, they're a natural for smartphone typing.

With these formats, you also have control over who can correspond with you, which you usually don't in e-mail. And especially on Facebook, instant messaging can take on the character of a chat room, where several people can carry on at once.

As a bonus, none of these channels have been overrun by spam, and advertisers have yet to blast us that way.

Does this mean e-mail is on its way to the dustbin of digital history? Was it just a transitional technology—from postal mail to the new, rapid-fire communication channels?

Not necessarily. E-mail still has certain advantages. Whereas tweets and texts feel ephemeral—you read them, then they're gone, into an endless string—e-mail still feels like something you have, that you can file, search and return to later. It's easy to imagine that it will continue to feel more appropriate for formal communications: agreements, important news, longer explanations.

So, no, e-mail won't go away completely. Remember, we've been through a transition like this not so long ago: when e-mail was on the rise, people said that postal mail was dead. That's not how it works. Postal mail found its (smaller) niche, and so will e-mail. Technology rarely replaces an institution completely; it just adds new avenues.

E-mail down, messaging up. Now go clean out your in-box.