AO3's work numbers don't count up directly anymore - they use a randomizing function that makes the work numbers about 4-5x as high as the total number of works. (It would have over 50,000,000 if we went by work numbers.)
It looks like ff.net, at least in the early days, did just count up? But the work number was still significantly higher than the number of works. The only stats on # of works on fanfiction.net I could find were from 2010 (when the site was pretty much at its peak of relevance) and showed that the number of works could be approximated to about 60% of the highest work number. So if you had a work at around 2 mill in 2004, there were probably something less than 1.5 millions works available. Still a lot higher than my estimate! Still about 1/10 of what's currently available just on AO3 (ff.net is currently at about 14 million in work numbers, and Wattpad makes stats even harder to get but it's within the same range as those two.) So I think it's still valid to say that there's something like 10x as much fic available as there was in 2004, and it's much easier to find.
Re: Losing Yuletide
It looks like ff.net, at least in the early days, did just count up? But the work number was still significantly higher than the number of works. The only stats on # of works on fanfiction.net I could find were from 2010 (when the site was pretty much at its peak of relevance) and showed that the number of works could be approximated to about 60% of the highest work number. So if you had a work at around 2 mill in 2004, there were probably something less than 1.5 millions works available. Still a lot higher than my estimate! Still about 1/10 of what's currently available just on AO3 (ff.net is currently at about 14 million in work numbers, and Wattpad makes stats even harder to get but it's within the same range as those two.) So I think it's still valid to say that there's something like 10x as much fic available as there was in 2004, and it's much easier to find.