If it was exploitative, then presumably the users would not choose to use libra.
Unless one actually believes in the existence of homo-economicus and also has never previously encountered a history book, I have no idea how someone would draw such a conclusion.
Specific cryptocurrencies on the other hand, well, where do I start? We could be here all day.
1. All the interest from the reserve (which isn't all hard cash but also low risk securities) goes to the investors. No ability to create your own bank offer interest and compete.
2. Undercuts sovereignty. The HN crash may laugh but ask Yanis Varoufakis what it's like when you can't do your own monitary policy. Would you rather deal with the IMF or Facebook?
Now that is a genuine dilemma. I'd say the IMF, purely because there is enough historic data to use Bayesian inference to guide you in doing the opposite of what they advise.
edit - Better the devil you know, in other words.
A single person might use Libra because it is useful for them, and one more user of Libra will not change much. However, it is difficult to assess the risks that could come with FB having control over millions of people's transactions, and then factor that in your individual decision. And that assumes that people know what can go wrong with monetary policy, which is far from reality.
To make an analogy... "If a country's citizens chose to use opioids for pain management, it must be because they found it useful, and the country should not have permission to prevent it."
So apparently no one does meth, or gets involved in pyramid schemes, or buys ridiculous ICOs.
You are right, there are no systems that are perfect when they involve humans. However, you used two countries that are, respectively, failures of socialism and communism as an argument against the "invisible hand of the market".
Also, I country might have laws about equal access to financial services, which libra might not offer.