[rectang]

The two things I generally think about with regards to sponsor influence on software foundations are:

1. If the sponsor is a software vendor, does the funding enable them to secure a marketplace advantage over competing vendors? For example do they gain a majority on a steering committee, or fund a critical mass of foundation-employed developers?

2. Does the funding enable the sponsor to unduly influence an official or de facto standard which impacts a marketplace outside of software? (For example, web protocols: see https://httpd.apache.org/ABOUT_APACHE.html#why-free )

I think you've properly identified "good will and the dependence on funding" as sources of potential undue influence. The structure of the foundation is supposed to guard against that. But more to the point, it's hard to imagine how Facebook would be able to leverage undue influence over the Rust Foundation: they aren't competing against Rust vendors, and it's unclear what public standards could be subverted via the Rust Foundation in a way which could benefit Facebook.