A report released yesterday based on Nielsen Mobile data compared user demographics and market penetration of Twitter, a free mobile community, and Predicto, a paid mobile community. 

According to Nielsen’s fourth-quarter 2008 SMS Media Report, both services enjoy an overwhelming 50 percent of their respective community market share in terms of SMS activity. 

For the fourth quarter of 2008, Twitter amassed about 812,000 unique SMS users, while Predicto Mobile interacted with more than 2,303,000 unique users. 

The data also shows that Twitter has a dominant presence among young and male-oriented audiences while Predicto attracts a more mainstream following with a broader penetration, particularly with the female and older demographics.

But while those numbers are impressive, Nielsen also released a study that shows Twitter might be having some retention problems.

According to a recent Associated Press report, more than 60 percent of Twitter's U.S. users don't return a month later, based on an analysis of traffic trends unveiled this week by Nielsen Online.

The AP also reported some criticism of Nielsen's conclusions. Twitter fans said the research firm didn't attempt to account for applications on mobile phones and other devices that bypass the Web site.

As a result, Nielsen revised its methods and still showed a 60 percent churn rate.

According to Nielsen, Twitter is losing users at a quicker pace than Facebook and MySpace did at comparable stages of their growth. Both Facebook and MySpace were retaining about 60 percent a few years ago and now are holding on to about 70 percent of their traffic.