Presumably to sell you ads, which Facebook doesn't consider selling your data--it's just selling access to you, which is only valuable because of the data. Does it really matter if they didn't sell your information if they just used it to show you the same ad they would have showed if they had sold your data? It seems they're relying on the idea that advertisements are somehow an OK form of privacy violation.
On the surface, that's actually a quite meaningful distinction.
But, less so when you consider that businesses go in cycles, and one day Facebook/Google/whoever could get desparate enough to decide they need some extra cash... or they could go bankrupt, and have it auctioned off...
Data collection as a whole is certainly a booming market, but it allows facebook to bypass the very reasons they were called into congress: mass manipulation. Facebook is a platform of mass manipulation. Ads are just the polite name. All of us realize that CA is a small fish compared to the other data warehousing companies out there. Does congress?
Targeting ads is the tip of the iceberg in terms of what could be done with the data if Facebook/Google/whoever gets more desperate for revenue.
https://techcrunch.com/2016/05/26/facebook-starts-selling-of...
If you're visiting a site or using a mobile app served from facebooks ad network, you don't need to be on facebook or have a facebook account to be shown an ad.
1. Facebook runs its own ad network, so you might see Facebook ads even if you never go to Facebook's website.
2. You might still go to their website even if you don't have a Facebook account. Many local restaurants and community organizations only have a Web presence through their Facebook pages.