In my Scientific American column this month I noted that humanity's flood of e-mail seems to be subsiding. The quantity has dropped 10 percent in the last few years, and among young people it's dropped a staggering 60 percent.

But that doesn't mean that written communications are dead—far from it. It means that we've found better, quicker, more targeted channels for sending messages, thanks to our trusty smartphones and tablets. With the rise of these new apps and channels, two key aspects of e-mail are changing: the store-and-forward routine (I send you a message, which waits until you come and get it) and the fact that these communications are typed.

Here's a rundown of the messaging channels that are eating away at e-mail, and why:
Text messages (24 billion a day): Pros: Fantastic compatibility—built into every cell phone on Earth. Easy to send. Can include photos, audio clips or video clips. Cons: No text formatting. Severe length limit (160 characters on some). Often no indication of when your message has been received or read. Costly on some cell phone plans.

Facebook messaging (216 million a day): Pros: No minimum or maximum length. Can include photos, videos, documents or links. Your address book is built in. Very little spam. Incoming messages pop right up on your phone or tablet. Cons: No easy way to format your text. Recipients must be Facebook members. In fact, recipients must agree that they're your “friends”otherwise, your messages go into an "Other" folder that most people don't even realize exists.

Twitter (500 million a day): Pros: Concise. Messages can be either public or private. Not too much spam (yet). New messages appear instantly on your device. You have a huge range of choices for apps, sites and programs to use for sending tweets; in (little-known) fact, Mac and Windows both have built-in features for tweeting. Cons: No text formatting. You can't send private messages except to people who are "following" you. Severe length limit (140 characters). Messages scroll away very quickly if you have a busy inbox.

Apple iMessages (five billion a day): Pros: Unlimited length. Text, photos, videos and audio are all easy to choose or to record on the spot; file attachments can be huge; text formatting available. Clever symbols show you when the other guy is typing a reply or when he's read your note. All of your conversations show up identically on all of your Apple gadgets. Cons: Limited to Apple devices (Macs, iPhones, iPads, iPod Touches). No per-message charges.

WhatsApp Messenger (50 billion a day): Pros: This free app is available on every kind of smartphone (iPhone, Android, BlackBerry, Windows Phone). Messages can be any length; text can be formatted; photo, audio and video attachments are easy. Read receipts, customizable backgrounds, easy group messaging. Cons: Messages show up only on one device (your phone). Most popular overseas. No tablet or computer version. Both parties must have the app installed.

There are dozens of other messaging apps, each requiring the other party to have the same app, but WhatsApp (which Facebook bought last year for $19 billion) is by far the most popular.