Sloan, L., Morgan, J., Burnap, P., & Williams, M. (2015). Who tweets? Deriving the demographic characteristics of age, occupation and social class from twitter user meta-data. PloS One, 10(3), e0115545. doi:10.1371/journal.pone.0115545
This is a paper by the COSMOS group, and starts to fill a key gap in knowledge that is needed to use Twitter as a research source.