1. Snapchat is not done. Millions of active users in advertiser-desired demographics is not done. Instagram has blunted its growth, but Snapchat is still alive and well.
2. Snapchat is a social platform, not a broadcast media company. There is a difference- they're not making their own content.
3. Snapchat was always a threat to Facebook, regardless of whether it was 1:1 or 1:many messaging. Snapchat competes for finite resources: digital ad dollars and user attention by way of socialization. This is how Facebook's business works as well, making them inherent competitors. If you're snapping a selfie to a friend then you're not seeing ads in the Facebook app.
You may be right about Snapchat not being able to compete against Facebook in the long run. Facebook certainly demonstrated the power of owning the largest social network when they cloned Stories. They can copy good ideas and roll them out to billions of users instantly. Maybe Snapchat can compete with that, maybe not.
4. Exactly what Snapchat's turf consists of is an open question I don't think any of us know the answer to. You seem to think its turf is 1:1 messaging, but Spiegel thinks it's being a camera company. After seeing Spectacles, god only knows where they'll go next. Besides, why stay in one bit of "turf" when you can expand?