Ya, good question! The CNBC article makes it sound like it's just a coin to send back and forth. It's possible they didn't do a great job at explaining it tho.
A way to reward people for using Facebook would be more interesting. Maybe it will be something like..
They're making it where if you hold Libra, it's easy to click a couple buttons and make a small tip to a post you like.
That's essentially how STEEM works, just in a super convoluted way (like if you hold SP >>> you hold the rights to some newly inflated tokens >>> which you can send to people with an upvote.. so it's essentially also a tipping mechanism, just so convoluted that many people don't think of it that way)
I admit that I'd probably think about Facebook more if tipping for good posts became a thing.
But I'd rather they used BTC or USD. Build the tipping mechanism using one of those, rather than create Libra.