As of December 2008, 11% of online American adults said they used a service like Twitter or another service that allowed them to share updates about themselves or to see the updates of others.
Twitter and similar services have been most avidly embraced by young adults. Nearly one in five (19%) online adults ages 18 and 24 have ever used Twitter and its ilk, as have 20% of online adults 25 to 34. Use of these services drops off steadily after age 35 with 10% of 35 to 44 year olds and 5% of 45 to 54 year olds using Twitter. The decline is even more stark among older internet users; 4% of 55-64 year olds and 2% of those 65 and older use Twitter.
Read about the new report from Pew here. The last section speaks to the importance of mobile communication platforms and open APIs:
Overall, Twitter users engage with news and own technology at the same rates as other internet users, but the ways in which they use the technology — to communicate, gather and share information — reveals their affinity for mobile, untethered and social opportunities for interaction. Moreover, Twitter as an application allows for and enhances these opportunities, so it is not so surprising that users would engage in these kinds of activities and also be drawn to an online application that expands those opportunities.