Yeah so unfortunately that's not how it will work in practice.
If facebook prioritizes certain kinds of content, that stuff will inevitably be what gets shared and commented on more often, and given the size of the potential userbase, they'll effectively be able to set the tone and the topics for the wider network. Plus, you'll find you get more engagement when fb boosts your stuff, so in this way you'll be encouraged to 'dance to their algo' as I like to call it.
In terms of surveillance: at present, your social graph here is separate from whatever social graph fb has on you on their platform. Once federated, they can merge these two data sets, making whatever data they may have grabbed from the Fedi far more valuable.
In addition, due to the fact that you'll often need to click on a profile to view it "on the original page" means fb will quickly be able to link your fedi profile with their wider web surveillance apparatus.