I think we can say three things fairly uncontroversially in favor of this
1. The world could use an online independent currency
2. Adding stability to blockchain currencies and having that work on a large scale is a good thing
3. Unlike government issued currencies, any monopoly or control facebook derives isn't done through force, it's by making a coin better than all the other coins. Other people are still free to make their competing coins.
That said I understand the detractions that many here are presenting. I just wish there could be a deeper exploration of both the pros and cons.