I think you should consider it ethical to do ethical things using a tool created by unethical people. Consider for instance Fritz Haber, the so called "father of chemical warfare" who contributed to the development of the Haber–Bosch process for artificial nitrogen fixation, that facilitated the mass production of explosives by Germany during the Great War, but also provides as much as 50% of the nitrogen in the body of an average human today due to it's role in the production of fertilizer.

Whether it's ethical to contribute back to the project knowing that the unethical creator might derive unethical utility from your contributions, is perhaps slightly more complicated. However the same could be said of any open source project, you could create something new wholly from scratch and if you release it publicly, somebody else could use it for something unethical.

I commend your consideration of ethical concerns, which I think is lacking in the tech industry today. But in this particular case I don't believe there is too much cause for concern.

I think your points generally stand, but I think Facebook open source raises some specific issues at least worth consideration:

If your use of it contributes to its popularity, perhaps becoming the standard of X area, does that give Facebook the company more power and possibly enable other unethical actions?

I think it's probably not as much of a worry given the narrowness of the area, but I do think this is something to consider when it comes to React for example.

No

why not ?