The number of Facebook users in the US will increase 13.4% this year, eMarketer estimates, after 38.6% growth in 2010 and a whopping 90.3% rise the year before. The rate of adult Twitter user adoption has similarly begun to plateau, dropping from 293.1% growth in 2009 to 26.3% this year and still slowing. In many developing countries, these and other networks are seeing their audience growth taper off as most new users come from other countries such as the BRIC nations and Indonesia.

Meanwhile, users in more advanced countries have been shifting their behaviors after spending years on the sites. According to the GlobalWebIndex “Wave 5 Trends” report, social network usage growth has all but stopped among 16- to 24-year-olds in the US, and in a few countries usage within this already-saturated group is actually declining.

Among those who remain on Facebook, GlobalWebIndex reports, there were declines in participation in activities like messaging with friends, sending digital gifts, installing applications and joining groups between July 2009 and June 2011. The activities on the wane are decreasing faster in the US than worldwide, and are often decreasing even further among college-educated US users under the age of 30.

Meanwhile, on microblogs like Twitter, the heaviest users are focused on disseminating content. Links to other microblogs, personal photos, and links to videos and news stories were the top subjects of status updates on these real-time oriented social sites among frequent users. Other than personal photos, these all relate to content created by others, while most content creation activities scored lower.

The report also noted the high demand for professional content. Traditional sources of news were dominant, including among microbloggers and heavy social network users. And when asked what they want from brands, consumers ask for knowledge and, among younger adults, entertainment. Brands have an opportunity to use the transmission-oriented social media landscape to disseminate valuable content to followers—who in turn are hungry for interesting and entertaining content to transmit.