Yes, I would guess that the primary function of Farcebook - is to reap data.
Data is or soon will be, after all, the final currency.
What it does with the various data which is gathered is up to the owners of the data.
The users have already agreed to this, by taking part.
There's no point crying about it - I would say either use it or dont - but I think once you close your account - they can still retain the data you have supplied them with - although I've no idea on what terms and for how long.
In the most part I would guess that this data is analyzed en masse. A little like Hari Seldons mathematical principle of Psychohistory from the Asimov novels. - But of course, there is no reason at all why individuals can not be singled out for 'analisys'
- or groups, or types, or sorted by age, race, religion, political persuasion, favourite football team, fascination with the colour blue, left brain/right brain orientated, movie goer, aggressive, passive, attractive - you name it.
I would be willing to bet that various advanced 'Narrow AI' applications are employed in the scanning and interpretation of data, collected via Farcebook. As AI becomes more advanced more subtle extrapolations will be possible. These extrapolations will become very valuable information. If you require custom extrapolations - for a new product or branding etc - then the data is already stored - and the AI can re-analyze using the impetus you supply !