This sounds like somebody planted a seed of FUD about snapchat that ignores what's a bigger question in my mind -- are the users who typically choose Snapchat now choosing Facebook?
I don't believe alot of Facebook narratives. Most of the people in my social circle count as part of their 1B messenger users, but all of their substantive communication is happening on iMessage. When you have billions of users, it's hard to make definitive statements about network effect, as even small variations are big. Facebook's network effect doesn't seem to have as much mojo as Microsoft or AOL did in the old days. Hell, AOL had millions of people paying $20/mo for a decade for nothing.
Per Google, they have 1.86B monthly users. Per this article, Only 60% of Facebook users use messenger, Only 1/3 of Facebook users use instagram, Only 1/3 of Instagram users use stories. That's pretty telling to me, especially considering all of the ways they trick you to click on things and gin up the numbers.