Here's (imo) the most obvious reasons.
Political Power- We used to complain about Rupert Murdoch's power. Media and journalism are a leg of democracy, so having a corporate oligopoly controlling it was worrying. Zuck's influence is way bigger. There have already been a handful of revolutions that started on FB, a dozen attempts at revolutions, full scale coups and thousands of elections where fb played a bigger role than any past media company. They didn't even notice this until it was Trump and/or stuff FB-ers cared about personally.
(1) This is too much power. (2) FB are pretending this has nothing to do with them. They don't care about journalistic integrity, or feel it applies to them. They let anyone with a credit card sign up and run political ads. extremely irresponsible, at best.
Monopoly - Thiel put it best, startups are about monopoly-or-bust. Uber wants to be the only ride app. YouTube the only free video site. FB the only networking app.
The pattern is often: Innovate. Win. Then milk it with ads and spying and/or stop innovating. YouTube for example, it's nowhere near as good as it could be, but why bother.
Data - data gets different qualities as it grows. A lot of the creepiness of targeted ads have more to do with how many people's data they have, not how much data they have about you. It locks in monopolies. It's also the newest category of "IP," which is (imo, but not gonna support it here) a current and future driver of wealth disparity.
We're now (definitely in Europe) in a period where the public/legislators are very keen on regulating. This will likely lock these monopolies further into place. 4 out of 5 times, regulation is incumbent friendly.