Pew Internet Research has been busy looking at the demographics of social media use in the US.
Firstly, Pew should know better than to lump all of the older age group into a single bucket called the 65+. There will be a big difference in use between somebody aged 65-75 and somebody aged 75+.
The numbers pretty much speak for themselves. Worth noting that Facebook use has pretty much spread throughout the income/socio economic groups and there is not that much difference between rural and urban.
The research also looks at the use of Twitter, Pinterest, Instagram and Tumblr.