You don’t think there’s a difference between people who signed up/registered/interacted with Obama and just buying 50 million records from a personality quiz that was on FB 4+ years before?
http://www.dailymail.co.uk/news/article-5520303/Obama-campai...
If you'd like a more in-depth explanation, I found this post by the former Chief Scientist of Obama For America campaign (2012) that goes into the key differences between how the data was collected & used compared to Cambridge Analytica: https://medium.com/@rayid/why-what-cambridge-analytica-did-w...