If you watch Jonathan Morgan's presentation, you'll see that he says that his favorite theory about the decline in active editors after 2007 is the rise of the popularity of Facebook.
It is in a sense true that the Facebook Theory is my favorite theory about the editor decline. But that's because that particular theory, of the many theories that exist, is singularly unverifiable and unrepeatable. So it's fun to see people trot it out in debates about the editor decline (as many do), without the need for pesky things like evidence.
My actual favorite theory is a dog's breakfast of rising mistrust of newcomers, bureaucratization, and a lack of diversity along multiple dimensions.[1][2][3]