App icons are displayed on an iPhone.
App icons are displayed on an iPhone. - 
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There's an app for that. So stop using the website.

This is the message companies are trying to drum into users. Mobile is, as we know, convenient and personalizedbut there is more to it than that. Apps let companies mine far more of your personal data than websites. That is a major incentive to get you to make the switch from web to mobile.

If you try to use the real estate website Zillow on your phone, you are bound to run into a full-screen ad for one of its many apps.

“Now,” says Jeremy Wacksman, who leads mobile strategy for the company, “Zillow can be present in your pocket when you're touring open houses, when you're driving around the neighborhood, when you're laying in bed at night checking something.”

With all its apps, Zillow can keep you company day and night. Wacksman says two-thirds of Zillow's traffic is now on phones. The company has also partnered with Google Now, an app that gathers information from places like a user's web browser, phone GPS and other Google products. Zillow wants to use that data to show people ads for houses before they even know they want them.

Mobile data opens up all kinds of possibilities for companies. For instance, your contact list can help Instagram connect more user accounts; LinkedIn can learn about your social network from your calendar. And, of course, advertisers will pay a pretty penny for that kind of data.

Every year Appthority releases a report rating the security and privacy of top apps. Company co-founder Domingo Guerra says there is so much more information on mobile than web.

“You have the GPS location, so exact coordinates maybe 24-7,” he says. “And then there is access to cameras, microphones, calendars, address books, even vibration of the device.” That is powerful data he says can be sold or leaked to third parties.

Even if your data is not distributed, Domingo says it is uncertain what companies will do with it all. Many of those companies don't know themselves.

“The more data they collect now,” he says, “the more uses they can find for it later.”

If you do switch from a website to its mobile app counterpart, it may not be obvious what information about yourself you are giving up.

"It's not standardized and it's not regulated," says Susan Grant from the Consumer Federation of America. "So the only way you can tell is by looking at the privacy policy. If it's not clear to you what information is being collected, then don't use that app.” Often, she says, it's not clear.

In any case, some companies do not leave consumers much of a choice. On Facebook, for example, mobile users can only send messages by downloading its new messenger app, which, by the way, has been criticized for gathering all sorts of personal data.