China‘s Internet population passed the half billion mark at the end of 2011 after the country added 28 million new users during the second half of the year.

At the end of December, the country had 513 million Internet users, according to a report issued Monday by the China Internet Network Information Center (CNNIC), a non-profit group with ties to the government. This puts the country‘s total Internet penetration at 38.3 percent, up 4 percent from a year ago. In contrast, the U.S. has a total Internet penetration of 78.2 percent, according to Internet World Stats.

The number of users accessing the Internet from their mobile phones has also grown, reaching 355 million, or more than the entire population of the U.S. Now 69.3 percent of China‘s Internet users connect to the Internet via their handsets, up from 66.2 percent a year ago.

Users typically use one or more devices to access the Internet, but the percentage of users relying on desktop PCs to link to the Internet has decreased to 73.4 percent, while the percentage of Internet users that access from their notebooks has remained steady at 46.8 percent.

The growth in China‘s Internet users has slowed in recent years, down to single digit percentage increases every six months. CNNIC‘s latest report noted that most Chinese people within the 10 to 29 year old age range already use the Internet, but that more work needs to be done in getting older population groups and people less educated to use the Internet.

Instant messaging, search engines, and online music are the most popular Internet applications among Chinese users. But the number of users of China‘s microblogs, which operate like Twitter, saw the most growth in 2011. Their numbers reached to almost 250 million users, up from 63 million a year ago.

Analysts, however, have said the statistics provided by the CNNIC are inflated. The CNNIC defines users as people, ages 6 and above, who have connected to the Internet in the past six months.