Straight-up text messaging is rapidly becoming old-school, The New York Times's Brian X. Chen reports today, as people begin to prefer what he calls "free" over-the-Internet messaging services like iMessage and What's App. That's good news for your phone bill, at least for now, since wireless carriers have been overcharging for sending messages since forever. Text messages cost phone companies pretty much nothing—about a third of a penny, per Time's Eric Bender—as they are "free riders," which The New York Times's Randall Stross explained a few years ago. Higher volume also doesn't cost these carriers any more money. Yet Verizon, for example, charges $.20 per-text, or a flat rate for a plan with unlimited texts. With 1.1 trillion messages sent last year, that's a lot of pure profit. Apps like iMessage and What's App, however, don't travel over the same tubes as texts, using data and WiFi to make their way from one phone to the other. That arrangement costs a lot less for users, but is it really free? Of course not. And it might not stay so cheap forever. 

An iMessage doesn't cost a user nothing, though it's a lot less than $.20 per message. It takes a teeny tiny bit of data to transmit each message, which users pay for via a data-plan. Apple doesn't say how much one iMessage uses, but Macrumors and Apple forum users who have talked with Apple employees estimate the following: 

Let's put it this way, if each message takes up 1K (and it's probably much less than that on average), and you send or receive 1 message every minute you're awake (16 hours a day) for a whole month, you'd use 29 MB for those 28,800 messages, or less than 15% of even the smallest 200 MB data plan.

Yes, pictures are going to use more data. You could send 1,000 (about 33 per day) 100K picture messages (again, that's on the large size, most .jpg pics will be smaller) and only use 100 MB, or 50% of your 200 MB data plan.

So, if it's $20 per month for 300 MB of data, and a message takes 1KB (or .0001 MB), or so, that equals $.000067 per message, which is very much cheaper than $.20 per message. That's for iMessage, a data-tracking research firm suggested that on average, What's App users spend 1.2MB per month, which calculates out to just 8 cents per month. The app also costs $1 to buy.

Those calculations depend on how much your cell phone company charges for data, which is the part that should scare you because it is subject to change. Already, companies have started altering the way they price data because of our new habits. The data share plans introduced by Verizon and AT&T over the summer did just that, for example, with higher data plans costing more money. Just as texting once fed huge sums of money to the phone companies, Verizon has posted huge revenue gains for data. These apps might cost just about nothing for now, but there are ways for companies to capitalize on them, such as raising prices on data plans, or creating a separate, "iMessage" only data plan, as Sascha Segan suggested over at PCMag. The other, less legal, option would be to charge these messages as text messages. Though, that's against net nutrality, as Segan points out. (But, has that mattered before? See: AT&T's FaceTime tyranny.) 

Thinking of these messages as completely free only makes it easier for phone companies to sneak in ways to make money off of them in the future. Ultimately, they hold the key to the tubes that make the communication go. And, they aren't about to let all those profits go.