I agree that it is technically illegal, but if the most recent example of a conviction (that you can think of) was almost 30 years ago that tells you that it's effectively unenforced.

Michael Cohen was sentenced for exactly this [1] just a few months ago.

[1] https://www.economist.com/united-states/2018/12/13/michael-c...

Yet James Clapper somehow got away with it. As do a lot of other people who have the same "political opinions" as Zuck.

Didn't that also involve lying to federal investigators?

Does it? One conviction in 30 years is also consistent with the conclusion that not that many people get invited to give evidence to Congress, and the people that do tell the truth (or at least don't tell provable lies).

Definitely illegal, not just technical.