(I have not read the Libra papers fully. Im doing a far speculation here.)
Well... let's say you're a politician wanting to regulate Facebook, and let's say you have an affair with whom you communicate on Facebook or in Instagram DMs. If I were Facebook, I'd simply let an investigation team check if any dirt can be mined out of the troves of data - anything, even the dataset of GPS or other location pinpoints, can be enough to get a first hint. And now I discretely contact you and advise that there may be information around and that it would be a pity if that information were to get public.
This is the ultimate problem with the size and capabilities that companies such as Facebook have.
That's Dark and Illegal
Is this really a threat any longer? I mean, Stormy Daniels, $130k payoff pre-election, etc and all...
That's not true at all. Cash has the same problem of undeclared transactions, but the tax mostly still manages to get collected. If anything, the fact that Libra transactions will leave an electronic record makes it more likely that tax will get paid.
It's axiomatic that cryptocurrency transactions are rarely taxed, most people have not paid taxes on those transactions. The IRS is certainly trying to tax them, and the electronic ledger function of (some) cryptocurrencies will make it easier to track them under some circumstances than cash. That may change as time goes on -- I'm not sure enough how libra is implemented to have an opinion on that yet.