It has been recently shown that the digital footprint of users can be used to automatically infer their demographics. On this page, you can find examples of exisitng work.
Personality, Gender, and Age in the Language of Social Media: The Open-Vocabulary Approach. A. Schwartz, J. Eichstaedt, M. Kern, L. Dziurzynski, S. Ramones, M. Agrawal, A Shah, M. Kosinski, D. Stillwell, M.E.P. Seligman, L.H. Ungar. PLoS ONE, 2013. The 10th most impactful paper of 2013. paper
Computational Personality Recognition in Social Media. G. Farnadi, G. Sitaraman, S. Sushmita, F. Celli, M.Kosinski, D. Stillwell, S. Davalos, M-F. Moens, M. De Cock. User Modeling and User-Adapted Interaction: The Journal of Personalization Research (UMUAI), 2015. paper
Computer-based personality judgments are more accurate than those made. humansby W. Youyou, M. Kosinski*, D. Stillwell. Proceedings of the National Academy of Sciences (PNAS), 2015. The 19th most impactful paper of 2015. paper
User profiling through deep multimodal fusion. Farnadi, G., Tang, J., De Cock, M., & Moens, M. F.. In Proceedings of the Eleventh ACM International Conference on Web Search and Data Mining (pp. 171-179). ACM. 2018. paper