Montenegro has a population of about 600K. Its annual GDP is $12 billion.
There are also small countries that are pegged to the US dollar.
When you are a small-potatoes nation-state, and have a non-diversified or small economy, it is a great idea to use a larger, better established, currency.
* It's more stable
* Trade is easier
* More than 3 banks accept your currency!
The incentives of the nations controlling the euro are well known to Montenegro, and they trust that those countries will keep the euro stable.
Libra is a whole different ball of wax. It's not just a currency. When you use it, you are tied to Facebook's terms of services, Facebook's desires, and Facebook's fate. Your individual citizens could be locked out of the economy for reasons that have nothing to do with your nation's laws.
If Google and Apple wouldn't agree to use Facebook-dollars for all their transactions, I don't see why a company would agree to.
There's a big difference between having a currency pegged to another, and not actually having a currency at all, and using physical foreign currency for everyday transactions.
Pegs can be broken and may be hedged with FX futures.
The second situation is a much stronger existential commitment, which makes it more reliable and predictable. Reinventing a new national currency is a high barrier, which cannot be hedged directly in the FX markets (although sovereign CDS might be close enough).
