> Since FB is a US company does that mean every single Libra user will be subject to US restrictions - even those which are legal in their country? Will Iranians be able to use Libra? What about Canadians using it to buy marijuana?

There is a long history of non-US banks which have operations in the US getting fined by US regulatory agencies because they violate US law somewhere else in the world with regard to money laundering / sanctions etc. The reach of US prosecutory power with respect to financial institutions is vast.