"However, over the past six years, we’ve also seen the user cost of switching between centralized communication services reduced substantially, particularly given the tendency towards addressing with user-owned identifiers like phone numbers."
Does he really believe that phone numbers are user-owned? My phone carrier owns my number. If I'm lucky, I might be able to take it to a new carrier. It's far more likely that I'd drop the number in favor of one in the area code in which I actually live, however.
I own my own domain, and I self-host my own email. I'm not likely to change it for geographical reasons. It has a much stronger tie to me than some string of digits used by an analog voice network from the last century.
The other user cost of switching between centralized services is network effects. If you want to switch from ICQ to AIM, you need to get all your friends to join you.