> "Network effect" refers specifically to "the phenomenon by which the value or utility a user derives from a good or service depends on the number of users of compatible products."
Sorry, I skipped a couple of logical steps.
Gmail/Google accounts are a namespace where the identifiers are persistent long-term, thus the ordinary network effect that email has is undiminished, unlike previous email providers where you had to change the account every so often largely to cope with spam. [0][1]
Also, Gmail participates in the associated network effect of every Google service that has one, and there are quite a few, though Google has a bad habit of killing them off.
[0] The idea being that you're trying to reduce the email network's utility for spammers to reach you more than you're reducing it for yourself to be reached by non-spammers, which is like chemo killing the cancer faster than it kills you.
[1] Spam per-se wasn't the only reason, of course. In the Before Times I had to change my address due to a few older relatives repeatedly setting in motion endless Reply-All storms / email chain letters, and of course I also changed ISPs occasionally.
But neither of those things are network effect.
"Network effect" refers specifically to "the phenomenon by which the value or utility a user derives from a good or service depends on the number of users of compatible products."