That doesn't mean it's going to implode. Zuckerburg is a ruthless and savvy businessman, with many more years of experience compared to Spiegel. There are a lot of people who are not as savvy as us and Facebook will continue to derive profits from those people.
Snapchat has a very high user engagement but only across a very specific demographic. Even my 80 year old mother uses Facebook, I strongly doubt she would would ever have any interest in Snapchat.
Spiegel would do well to take a few pages out of the elephant's book and put them to use: he's assuming that his own demographic is the only one that exists. The kid has hit a profit mine and now thinks he can predict the future - user engagement is not everything, user retainment counts for something too.
I don't know whether I believe it, but the argument for Snapchat is that it encourages user engagement. So if you're a photographer who wants attention, it's better to have someone's dedicated attention for a few seconds on Snapchat than it is for them to glance it while scrolling through their feed on Facebook.
"Yes, and in his email he seems to believe that his dick pics app is "revolutionizing personal communication". Ok guy."