For people living in drains in the third world it could unlock their access to online economies and by extension eventual cryptoness.
There's nothing that stops them from using crypto now, yet they choose not to, so even if they will adopt Facebook coin, they will be unlikely to switch to true crypto later, just like people today still use Google and not DuckDuckGo, Windows over Linux and so on. The only way Facebook will have an edge is if they will create marketplaces that use their coin or make existing ones adopt it, things like freelance sites, online auctions and other, but people need to have some good reason for switching from their current methods to Facebook coin, and I don't see what could it be.
The thing that is stopping mass adoption is a Ludite mentality, and a knockout interface that every one from the kids to granny can use.
If I wanted to explain to my grandmother what a cryptocurrency was, and the whole Blockchain and all of its implications, she'll fall asleep as soon as a say consensus. If I use an anology by explaining crypto using an interface like Facebook (which is familiar and tangible, from the young, to the old), the concept would be easy to grasp.
Picture what Facebook coin will do for the Instagram (Facebook owned) crowd. Instagram is quite the hot commodity, who hasn't had a guilty pleasure and fancied a glance at the yogapants gallery?
Forgive me for my immorality.