The USP of Snapchat was that parents could not go through past messages as these would get deleted once read. So, if you had planned a night out on the town doing silly things you could still pretend to the parents that you would being having a sleep over at a best friend's house. Even if they went through the phone with a fine toothcomb the parents would not find evidence of plans for wrong-doing.
Facebook is not about regularly messaging your best friends, it is about messaging/stalking people you knew from school onwards up to now, including workmates, parents and other people you don't message important things to, such as plans for tonight.
Snap are pretending here that their 'besties' market isn't already consumed by WhatsApp!, which it is, so Facebook have already eaten their lunch there. They don't actually acknowledge their 'secret sauce' in this memo, which was the instantly deleting messages that appeal to teens.