Facebook is part of the mass surveillance apparatus because it collects a vast amount of data on all its users (and even non-users) indiscriminately including locations, political leaning, etc... This is different from targeted surveillance where specific individuals are tracked for a specific purpose. This mass collection in itself is harmless, but there isn't far of a leap between mass surveillance and targeted surveillance, and law enforcement agencies have been enjoying working with Facebook to easily gather data on persons of interest, and this is where this mass collection becomes problematic.

So Facebook collects data as part of its advertisement business model, but the wide array of data points attributed to named accounts made it a natural fit for mass surveillance by the US government.

There is, of course, the other reason it's problematic, as highlighted by the Cambridge Analytica scandal, where spin doctors were able to target very specific political messages to very specific Facebook user subsets, unbeknownst to the rest of the users and the media, but while I believe open social media protocols can be enforced, I'm not sure how you could enforce a ban on "unethical secret mass opinion change".