The second wave of a USA Touchpoints study has some interesting numbers on how widespread the reach and use of social media is.
The numbers suggest that social media use broadly (across all devices) is used at least weekly by 40% of U.S. adults - about the same percentage who use DVRs at least weekly. Computers remain the most widely used device for accessing social networks. Smartphones come in second, with a combined reach of about 23% (roughly equally split between those using dedicated apps and those accessing through mobile Internet interfaces. Tablet use, within this sample, combined for about 6% of social media weekly reach (also evenly split between apps and mobile Internet).
The report predicts that mobile use will grow most rapidly, as more people buy and use mobile devices, and as social media platforms continue to invest in, and promote, mobile applications.
Source - The Whole Story - Social Networking - The Hierarchy of Devices, OnlineMediaDaily
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