The code being open-source accomplishes very little in this case. It's what they do with your data after they get it that matters. Data is the new oil, and if a for-profit company can get it, rest assured they will use or sell it.
I get where you're coming from but data is the new oil in conjunction with machine learning/ big data algorithms. Thats the true evil. that part at least will be transparent with the open-source model IMO
Once they have the data, they can do whatever they want with it. They can run it through closed-source ML, they can sell it to other companies. I suppose people would know if they meddled with the news feed order, but that's about it.
Thats rather cynical... Sure!! The team of privacy and security activists backed by internet hall of fame member Phil Zimmerman who created PGP and fought the US govt are all actually closeted evil capitalists just out to get everyones data..
