providers came and went and custom domains for emails were extremely rare... at least in my circles
Not at all. On the consumer level they were typically associated with your ISP account. So you'd get to be <version-of-your-name>@<isp-net.com>
My parents still have the email they set up with the minuscule local cable company in the early 2000's. That's only the second email they've ever had outside of the dialup provider email they had in the late 90's (when internet finally came to our small town)
thats how it was in my circles until ~05-06. everyone kept switching emails and used a different address on each service/forum. until gmail came along. suddenly everyone used their gmail address for everything. and even that got phased out when facebook came along.
Social networking was NOT an enormous pain in the ass. Everybody had, and still has email - it was the gateway to the internet. Roughly since the beginning of the internet. And all the people that joined. Email got used in almost all of the ways that Facebook did.
In addition to email, there were ways to do "roughly" everything Facebook did with varying levels of difficulty - some roughly unobtainium for the "later billions" of people that Facebook drew - not just email based listservs, but USENET before that, self hostedweb forums starting in the 90's, Craigslist in the commercial space.
The problem IS Facebook (and in a different but related way, Google).
UNlike Craigslist (for example), Facebook is a public company. But it is not accountable to the public, as we've seen over and over again. It's users are a commodity and are not stakeholders in any meaningful way. But - even the shareholders are not. Zuck owns AND controls it almost entirely. And unlike Craig Newmark (who isn't perfect), Zuck appears to have been deeply flawed from the very beginning, and the poison of unearthly money and power have only made it worse.