Worth noting that Gmail also 'solved' spam, which was the strongest reason to change email addresses/providers.
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."
> "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.
With Facebook, it doesn't matter how much you dislike the company or how good some competitor is. You still can't talk with people on Facebook (and often, even view content) without being logged into an active Facebook account.