What are some sites like meetup.org?

How do sites like alexa know what kind of people are visiting the site?

• How do they know details like education, browsing location (home, school, work)? I know that alexa has their toolbar but there are also other sites which provide this kind of data and they don't have user which have installed something like a toolbar. I am asking because I have to implement something like that myself. I know that the browser sends information like screen size, plugins enabled, IP etc. but with that kind of information you can't tell for example the education level of the visitor.

• Answer:

  KC pointed out that websites can use your IP to find out certain information about you. This is true, but something like knowing your education might be a bit of a stretch to get via IP. The more likely answer would be that they have used cookies, or some other equivalent system. Google does this, thats how they know what ads to give to you!