Growth in the volume of text messaging is slowing sharply, just as new threats emerge to that lucrative source of wireless carrier profits.

While U.S. cellphone users sent and received more than 1 trillion texts in the second half of 2010, according to CTIA, a wireless industry trade group, that was just an 8.7% increase from the prior six months. It was the slimmest gain since texting exploded last decade.

Text traffic will come under more pressure in the months ahead. This week, Apple Inc. showed off an application that will allow iPhone and iPad owners to bypass carriers and send text messages over the Internet to other people with Apple devices.

Google Inc., whose Android software is the most popular operating system on smartphones, has also recently worked on a messaging application, a person familiar with the matter said.

The new messaging tools—answers to Research In Motion Ltd. 's popular BlackBerry Messenger—are a growing threat to a texting business that generated $25 billion in revenue in the U.S. and Canada last year.