They get that you use site X (and y,z, and q too) , when you use those sites, and where you were when you logged in from(i.e. your IP, browser info, etc). For

They get that you use site X (and y,z, and q too) , when you use those sites, and where you were when you logged in from(i.e. your IP, browser info, etc). For one site, not a huge issue maybe, unless it's ilovemesome<insert something disgusting here>.com But add this up across many, many sites, and they suddenly get loads and loads more information to sell ads to you with.