I would argue that GitHub should remove this software. It just runs counter with their TOS.

A PR isn't the only way to contribute. You can also check out the repo and send in patch files.

Just wondering: for lazy repo bookmarking, why fork rather than star?

The issue people are (rightly, imo) pointing out is that forking without modifications—say, to remain locked to a particular commit for whatever reasons one may have—appears to directly violate the terms of the custom license (which itself appears to directly violate the terms of Github's TOS.

Yes, but forking on GitHub means that initially it will be just their software.

I read it as only allowing you to redistribute unmodified copies, because clause 3 only permits modification "for your own internal use."

No. Call that "source available", "source access" or something, but "Open Source" is pretty much established to be defined by the OSI definition of the term: https://opensource.org/osd

this might be silly, but why? what does Facebook gain by getting this library user adoption (beside some bug-reporting). I would think that for fb user adoption of the open source projects is not the goal, but getting OSS developers attracted and getting fixes/features etc. developed from them. Or is it some kind of pride? Fake internet point for having repositories on github?