Teens and 20-somethings liked the fact that Snapchat was their smaller circle of real friends. Facebook has that goofy uncle, and coworkers from 5 jobs ago, and old classmates you don't really want to stay in touch with. Facebook has too many "weak ties".
Therefore, Facebook could clone every feature of Snapchat but it can't change the fact that it is still Facebook. The new features might keep Facebook users from switching to Snapchat but would it entice Snapchat users to switch to Facebook? Sure, people can use social networks for a myriad of reasons but Snapchat the network seems to be a dominant factor.
Facebook is probably hoping that similar will happen with the main app.
It's definitely popular in high school. I know a police officer who works in a high school. Snapchat is a huge pain point for them. Someone takes a screenshot of a nude their girlfriend sent and passes it along to friends 6 or 12 months later after they broke up and suddenly 45 people have child porn on their phones. One person took a picture of a friend and in the background they noticed a student upskirting a girl.
I think the former is at least as important as the latter. Facebook has an order of magnitude more users than Snapchat. FB needs to keep their platform feeling modern and interesting so existing users don't feel the need to leave for greener pastures.
You can choose who you share your stories with on Facebook.
But Facebook doesn't need to get _all_ Snapchat users to switch. It only needs a tiny percentage of all those users and will severely damage Snapchat (by slowing down its growth). See this article for a good argument: http://www.businessinsider.com/facebook-copy-snapchat-2017-3
Sure, I get that from a pure functional perspective. However, I'm talking about psychology of app usage.
Consider an example of an iPhone home screen. The user sees the Snapchat icon and a Facebook icon next to each other. I believe that many people (millions) will mentally compartmentalize the yellow ghost as "my real friends" and the blue "f" as "every random acquaintance that ever sent me a friend request and I never said no because of etiquette."
Usage of each icon evolved to become its own "world" so to speak. Sure, one could theoretically create sub-worlds inside of Facebook but a customized "real friends of Facebook inside of Facebook" seems to be more work than just pushing the button with the yellow ghost. Staying inside the Snapchat world also minimizes accidentally sending the wrong Facebook-story to the wrong Facebook-friend.
It literally does have that option - go to their profile, click the "friend" dropdown, change it from "friend" to "acquaintance", see that they stop showing up in your feed :)
How does that work out for you? I could never get Facebook to give me a usable news feed. I unfollow friends, pages, like new pages, intentionally scroll through new pages and like random things in order to get its posts on my newsfeed, adding people as my "close friends", blocking random pages that are on my newsfeed for bullshit reasons ("John Doe and 2 other friends like this post") and I could still never get anything even remotely useful, neither by adding the data, nor by reducing the number of data.
The latest pitfall I'm having is "a video that you may like". It started appearing like less than seven days ago, and in every single example, it's just some bullshit video from a page that I have never liked, but have interacted with in the past once or twice.
A large percentage of my feed is from pages for video games and tech companies/software. It's possible my interests align better with how they select stories for my feed than yours?
Facebook has become the Blackboard of the internet; kids will use because it's a quick way to talk to their college friends and coordinate events and shit and that's it.
> Snapchat servers are designed to automatically delete Snaps after they’ve been viewed by all recipients. Opened Snaps typically cannot be retrieved from Snapchat's servers by anyone, for any reason. Also, Snapchat servers are designed to automatically delete unopened Snaps after 30 days. However, unopened Snaps sent to a Group Chat are deleted by default after 24 hours. 
"You grant Snapchat a world-wide, perpetual, royalty-free, sublicensable, and transferable license to host, store, use, display, reproduce, modify, adapt, edit, publish, create derivative works from, publicly perform, broadcast, distribute, syndicate, promote, exhibit, and publicly display that content in any form and in any and all media or distribution methods,”
In 2014, they were seeing 400 million images per day, 88% of which were to one person . Assuming 0.33 mb/image, no videos, and 100% going to 1 person, that's 132 TB/day.
Per their recent SEC filing, they're now up to 2.5 billion snaps per day. 
There's a strong business case for deleting the files as soon as you can. Before they were a gigantic company, their brand trust and keeping things lean encouraged deleting the photos. I believe that holds true today.
Edit: I decided to run some quick 'n dirty estimation and was a bit surprised by how "cheap" it would actually be to store it all. Snap uses Google Cloud Services, and given their volume, will have a pretty sweet pricing deal. For that reason, I estimated with $0.007/GB, which is the coldline rate per  even though they probably wouldn't be doing coldline storage.
Very rough estimate for 1 year of storing every image snap from now until next year is ~13.5 million. At 2.5 billion snaps/day, images only, it would cost ~$5775/month to store all the data generated today.
As an aside, I do wonder if there would be an opportunity for Snap to get and store context from an image without storing the image. It may be a balance between big data and minimizing costs. I'm thinking something along the lines of the list of words from Clarifai  stored with the metadata. It wouldn't be very useful to the police because the image recognition is wrong sometimes. "Marijuana" as a tag would be very weak evidence to put forth in court if the police didn't also have the image.
I am not convinced it makes sense for them to do it from an economic standpoint.
To an extent it makes even more sense in the Facebook app itself, but having the core app and Messenger have separate stories despite them using the same exact account is ridiculous to me.
EDIT: Stories also don't appear to show up on facebook.com.
EDIT 2: I just remembered that you can use Messenger without a Facebook account. Having them separate still doesn't make sense.
The people I see on Instagram aren't my small circle of real friends, or my bigger bigger circle of old acquaintances, but rather (mostly random) interesting people posting interesting photography of subjects I'm interested in. The Stories feature (temporary, unpolished) compensates perfectly for the deficiencies of the main Instagram product (permanent, curated), and vice versa.
Unless SC does something novel soon, I wouldn't assume that it's going to be around for much longer.
Isn't this what caused the dot-com bust of 2000?
In terms of adoption rate amongst people on my friends list, more people began using the Stories on Instagram immediately. Only 2 or 3 people out of 450+ "friends" on my FB have even posted to the story thing in messenger.
The first day Stories was available for Instagram, people were posting either the same things to both Instagram and Snapchat stories, or just regular "snap"-type posts.
If FB messenger adoption in my circle is anything like it is for the majority of regular users, then it won't be very popular.
Dropbox is doing pretty well though, if that's the comparison Snap might be in good shape!
That will be the day I finally leave Facebook for good, because I have no interest in downloading that monstrosity of a spy device onto my iPhone.
Maybe you can have it too by downloading Firefox for iPhone and changing the user agent. (I haven't tried but it's worth a shot)
Facebook's character fundamentally changes - again.
As it grows, I'm sure inertia will continue to slow it down, but that willingness for reinvention seems like quite a powerful property.
(Another current book – Antifragile (http://amzn.to/2nr15ST) – discusses systems that benefit from randomness and volatility. I wonder if the willingness for reinvention allows for a kind of anti-fragile generational selection to work: instead of waiting for selective forces to birth a different and new stronger generation, you transform yourself (or your company) to _become_ that new generation, allowing you to directly benefit from selective pressures. It's tough to do, psychologically and culturally, and the willingness to do so seems an extremely valuable quality to cultivate.)
Facebook measures and adapts to user engagement. When something spurs a loss of engagement eventually it disappears. In the social space, other tech companies struggle just getting a few new things shipped and then bail when they don't look like they are working out.
It is in Facebook's best interest to see Snapchat fail. The next big thing that comes around, it will be a lot easier for Facebook to buy early on.
On another note, if Instagram, Messenger, Whatsapp, and Facebook all start looking like the same app, it gives Facebook a particularly powerful platform. Everything could end up being rolled up in to a single app in the future -- or a new device.
Enough people use Facebook and either don't use Snapchat or use Facebook for other types of communication that SC couldn't really kill FB. But it would definitely hurt.
People who like snapchat?
Stories in particular launched earlier this year in Ireland, Chile, Greece and Vietnam.
> Facebook Stories, a feature that puts photos that disappear after 24 hours at the top of the Facebook mobile app, is rolling out to users in Chile, Greece and Vietnam, Facebook confirmed. However, it is not available to users in all markets. The feature was first tested among users in Ireland in January [...] 
It does feel a bit hacked onto the main Facebook experience though.
Specifically how? The only reason I don't use it is that Facebook has all of the users.
G+ has circles, which I really wish Facebook had. AFAIK, G+ doesn't have a constant sidebar with targeted ads like Facebook does.
I think it's because they are built on two different platforms that it would not make economic sense to interoperate.
Compare that to open source.
Honestly, I could see the same thing happening in open source, though you're right, it doesn't seem to very much at all.
edit: Facebook and Messenger may be linked. oops. Well, if you wanted to post to all Facebook products stories and Whatsapp, you'd have to post four times; tons of redundancy remains.
I get the appeal of stories and I may actually use it, but I think their shotgun approach is a bit too unfocused.
That's an annoying move.