IIRC this already happens and businesses can buy DMV databases.
> Is facebook going to torpedo their business by selling this info?
Many businesses collect and sell this information. Where do all these online businesses, who charge users nothing, get their revenue? [EDIT: More simply said, that info arguably is the basis of the Internet economy]
> why would they want it?
To discriminate. They may offer different products or prices. They may discriminate because you are gay or transgender, because of your ethnic background, or because of your religious (e.g., Muslims) or political affiliation (e.g., communist, right-wing extremist, etc.). This can happen in more serious situations too, such as when dealing with government, law enforcement, or in employment situations.
Certainly such discrimination has been widespread for centuries and is today. Why would it be any different when they have new tools?