While that may be true for regulations/ethics related appearances, this is not true for things such as funding. For example both Elon Musk and Bruno have gone before congress multiple times to discuss private space programs/ contracts and to justify their cases. Admittedly Elon did send Gwynne Shotwell several times but she is still the COO of the company.

More accurately there is risk in talking to the US congress when the topic sounds more like an inquisition than when congress is asking for opinions.

The matter then is why is it a risk for Facebook to discuss the CA issue? Are they worried about a witch hunt or a public ethics execution?

In this case there is user trust to be gained. By refusing to appear in front of congress, mark and facebook has lost a little of my trust.

Did they have any before?