Is this not exactly what Facebook, Google, and Twitter do with their tracking cookies? They figure out what you are interested in based on analytics regarding the sites you visit (obtained through multiple different methods), then sell that data to advertisers.
They sell access to those people. They don't provide specific identity information about those individuals. The Facebook example I posted shows how somebody was able to specify granular-enough parameters to focus on one individuals, but this type of targeting wouldn't work for everyone. I can imagine a day when this type of targeting is offered more-proactively.
No. Tech companies generally do not sell your data. They use your data internally to figure out what ad categories to show you. Advertisers can specify that they want their ads shown to people with certain interests. At no point does data about what you do on Facebook leave Facebook's control.
