It's a good business model ;) The issues with it are exacerbated by the promise they made to always be free (which makes it problematic to offer a tier of paid services even if users would be interested.)

They're getting around this by offering paid services that are outside their basic offering (services for business, messaging users outside your friend group, etc.)

See their post about always being free: https://www.facebook.com/facebook/posts/10150420085741729