iMessage, to be included in Apple's iOS5 update.

Sending text messages remains the single most common thing people do with their phones — well, other than using them as phones.

But the meteoric rise of mobile apps — including free alternatives to traditional SMS texting — is changing how many of those messages are sent, and shaking up the wireless world in the process.

Numbers released this week by comScore Networks demonstrate the trend. Among U.S. mobile subscribers, usage of downloaded apps has grown sharply over the past year — from less than 32 percent last summer to almost 40 percent as of this June.

By comparison, a whopping 70 percent of mobile users send text messages. But as explained by the Wall Street Journal, free apps such as TextNow and Apple’s upcoming iMessage promise to change how many people sent those texts.

Another sign of the trend: The next version of Microsoft’s Windows Phone will integrate texting with Facebook chat, letting users switch between them without starting a new conversation.

The WSJ says the alternatives threaten a major source of revenue for the big wireless carriers. Texting generated $25 billion in revenue in the U.S. and Canada last year.

Have you stopped paying for SMS and switched to a free app? We’re curious how many people have ditched paying a 20-cent per-text fee or a $20-per-month unlimited text messaging plan.

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