Monetizing on people’s relationships and social addiction, collecting and providing detailed data on all user activities, sentiments and connections, selling sets of such data to ...?

Advertisers. If you've ever bought an ad on facebook and see you can pick your target audience by which types of food they shop for or their income range from 10k increments alongside thousands of other filters.... fuck man they have a helluva ad platform

Not facebook's main functionality. The feature's functionality.

e.g if you are building a captcha test feature, it should not keep more data than is needed for it to function.

This is the policy where I work at least, and I believe this is shared by most privacy-conscious companies (which facebook is... as far as the legal definition of privacy is concerned :)).