> How to even prove it was done? The effect could be small enough to turn an election but not large enough to be reliably measured from the outside.

Facebook is still a company with many employees, and I'm quite sure even Mark Zuckerberg can't simply go rummaging in the code to favour himself.

In other words, there will have to be (quite some) people in the know about this, which makes it likely to leak.

("Requiring many people to be in the know" is basically what convinces me of the impossibility of pretty much every conspiracy theory.)