I am looking for public data sources about specific target groups. Although doing your own research probably gives the best results, having a kind of database with common target audiences would be a great help to get started and/or for budget strained projects.
Specifically, I am currently looking for data on media buyers. This question is more general, though, because I would love to build a resource list which can help with future projects and audiences.
So far I have found https://www.quantcast.com/, which gives you data on audiences of many websites. So if you know which site targets the same audience as your project, you can get some nice data from there. Another nice angle (if you are looking for data on users segmented by profession) is to check the profiles of job postings which often give a good idea about the education level & skills of the users.
Do you know any other sources of aggregated data?