I hope you take this as kindly as I intend it, but what you're proposing is a conspiracy theory. This is a relatively nice attribute for a theory to have, because it gives you a nice heuristic for deciding whether the theory is true!

The likelihood of a conspiracy being true decreases as the number of people with knowledge of the theory and an incentive to report on it increases.

To take an extreme example, if the moon landing was faked, tens of thousands of people have somehow held on to that secret. Tens of thousands of people who could gain overnight notoriety by telling their story, and hundreds would have the proof required to gain even more popularity. The fact that nobody has ever broken ranks is a strong sign that the moon landing was not faked.

"Twitter and Facebook are secretly tweaking which news stories Trump and the rest of us are seeing" isn't a conspiracy on nearly the same scale as a faked moon landing. It requires some pretty incredible things to be true though.

- Maybe every employee knows, and none of them have decided to say anything, despite the large incentives to reveal the secret and win their moment in the limelight.

- Maybe not every employee knows, just enough employees know to implement it and hide that implementation from the others. Maybe every employee on the Algorithmic News Feed team knows. I don't know how Twitter and Facebook are structured, the team probably isn't called Algorithmic News Feed, but as one of the more important systems both Facebook and Twitter must dedicate at least a hundred engineers. So, 200 people were quietly chosen for their ideological purity and ability to keep a secret from their peers. These 200 people write code in secret. Somehow they commit lies to the monorepo and apply private patches to the code before deploys. The SREs must also be in on it, because those private patches will still show up in traces and their bugs will show up as errors. All of this happens inside Facebook, a company notorious for employees who speak up and expect transparency. It also happens inside Twitter, a company with such lax controls that until just recently thousands of people could use the internal admin tool to take over any account.

I don't know, I guess it's possible? Maybe you have a better idea for how it could be happening, but it just doesn't seem very likely at all.