The forming of a financial underworld in the Eurozone is pretty doubtful in my mind. Sure, enforcement varies country by country but if this would get a big enough problem we also have international efforts in the EU to get rid of it. Worst case somebody gets a data dump in lieu of Swiss tax evasion dumps on CDs a few years in and the laws against it get even stricter.
Not true. It depends on the country, in the UK you can buy SIM cards from wending machines that work straight out of the package and it's not he only country that does not require ID for SIM cards.
Anyway, even if your identity is tied to your Facebook account, what stops you from creating an American or a Filipino Facebook account? Phone number? You can buy a phone number online. You can also buy a verified Facebook account too.
Let's say that Facebook somehow starts operating as strict as a governmental body and checks everything, what stops you from buying Libra from an American or a Mexican? I haven't checked the details but this Libra thing doesn sound like Steam game money but like cryptocurrency, so they probably cannot do whatever they want just like that.
Maybe in Germany, most people will stay away from it but in Bulgaria, Spain, Italy etc. - in places where people are more "relaxed" about the law enforcement will not miss business opportunities just because their rich neighbours can afford to miss it.
If audits materialize they, the users will simply set-up or use proxies to deal with it. Someone in the USA will set-up an Libra-Bitcoin-Euro-USD exchange for example and the transactions will go through these. I can already see an Ukrainian-Russian-Bulgarian-Romanian-Turkish partnerships to run these operations. Europe is a mess, laws and enfrcement capacity is very different in every country and countries are in multiple clubs(in EU, in EEA, in customs union, in Eurozone, in schengen, not in EU but confirms to EU, not in Eurozone but the currency is pegged to Euro, in EU but not NATO, in NATO but not in the EU. It doesn't have an end) giving differnt rights and different obligations to people in each country and even messier when they are not in their own country or hold multiple citizenships.