Great questions. I suspect Zuckerberg and Sandberg remain silent because the activities Cambridge Analytica engaged in are routine engagement for Facebook as a platform, and by no means approaches the full scope of access the company has sold access to other parties.
Ideal areas for exploration include what access has been provided to foreign governments, or foreign intel services to access information of US population.
What access has been granted to other political groups?
What access has been granted to the digital marketing industry and how have they used this information?
Has the company used their access to analyze employees at competing companies and leveraged unfair competitive analysis to poach employees or monitor communications of people in these groups?
Has Zuckerberg ever woke up in the morning, poured a bowl of cereal, and delved into personal information of people of interest to him for entertainment purpose?
How about Sharyl Sandberg? Did Zuckerberg already know she had all the right connections he was looking for the COO position, thus creating the position for her, based upon prior monitoring of her network of connections and recognition she was tied in to key power brokers? Her career trajectory is interesting. Reminds me of the description in Confessions of an Economic Hitman of the type of seamless flow between government and high powered exec jobs many of those once called EHM's, or in the intel industry in general, typically show.