They can seize money transfers en route to Facebook from their EU customers.
Facebook receives billions of advertising revenue from Europe (IIRC some 25% of their global revenue), all that money can be trivially seized by authorities, they just need to file the proper request.
Furthermore, there are international agreements on cooperation against tax evasion; I'm not aware of the details for this process but I assume that they could and would use the USA legal system to enforce collection of legitimate debts.
Facebook receives billions of advertising revenue from Europe (IIRC some 25% of their global revenue), all that money can be trivially seized by authorities, they just need to file the proper request.
Furthermore, there are international agreements on cooperation against tax evasion; I'm not aware of the details for this process but I assume that they could and would use the USA legal system to enforce collection of legitimate debts.