Facebook is the social giant, but Twitter is no slouch. Pew’s Twitter Update 2011 and data from Edison’s latest Social Habit report both suggest that the microblogging site can be a powerful marketing tool.

The Twitter audience is expanding. According to Pew, 13 percent of online U.S. adults use Twitter as of May 2011. This represents significant growth over the 8 percent of Americans who claimed they were Twitter users in November of 2010.

Edison’s numbers are slightly less impressive. The research firm indicates that 8 percent of Americans use Twitter, but it’s worth noting the company is including all consumers, 12 and up, in its report. Perhaps the microblogging site is more popular with the millennial and older crowds.

This seems to be the case: By Pew’s measure, nearly one in five internet users between the ages of 18 and 29 (18 percent) are Twitter users. Similarly, Edison says consumers between the ages of 18 and 34 make up 49 percent of all Twitter users.

Pew data also suggests that Twitter users are a more lucrative audience. The site is especially popular among Americans with household incomes of $75,000 or more.

Marketers who want to engage Twitter users must remember to post fresh content throughout the day. Edison reports that nearly one-third of users access the site daily, and 18 percent visit it multiple times per day. The company suggests Americans are becoming more engaged with Twitter overall, and the more engaged social consumers are, the more likely they are to share content. Eighty-five percent of “frequent social networkers” say they share content via Twitter, Facebook and other social channels.

In addition to publishing and sharing fresh content on a regular basis, marketers will want to offer Twitter followers direct responses to boost engagement. As Brafton has reported, 64 percent of Twitter users say they are more likely to make purchases from brands that answer their questions on the site.