Long time geek, fangirl, mother, and reader. I've got a lot to say, you might not like it all, but it will be honest and hopefully helpful.
"Megafandoms: Harry Potter and Twilight"
The Otakon (American anime/manga) convention just celebrated it's 20th anniversary and hosted 20,000 attendees. While Leakycon, the biggest Harry Potter convention in the US hosted 4,000.
"FanFiction.Net alone hosts over 500,000 Harry Potter stories, and there are many more on other archives, some still active, some gone and never coming back."
FFnet isn't used a great deal by most fandoms. In fact, it is often referred to as "The Pit" because of the perception that most of the fan fic on the site is of low quality. It also is avoided by many authors who writing explicit fan fiction because the massive takedowns that effect Harry Potter, among countless other fandoms. A fact, ironically, has not been mentioned in a section about Harry Potter fandom.
"...Fanlore lists thirty general Harry Potter fanfic websites (including some sites with more varied purposes, such as the influential MuggleNet, which include substantial fanfiction sections) and at least three times that many smaller archives devoted to particular relationships, themes, or characters."
A quick internet search yielded close to 50 anime/mange fan fiction (English) sites, over 30 sites hosting Buffy the Vampire Slayer. I haven't even mentioned Comic Book, video game, or sports fandoms.
The list of the fandoms with the most posted fan fics on AO3 demonstrates the use of particular fan fiction archive can vary from fandom to fandom.
Real Person Fiction 52,137
Harry Potter 49,383
None of this mentions the amounts of fan fiction hosted on LiveJounal sites that are difficult to track, much less calculate to compare. There's even an essay in this book on the Supernatural fandom that talks about how that fandom used LiveJournal to host a lot of its fan fiction.
All this aside, using the amount of fan fiction produced by a fandom as a measure of it's Megafandom status is deeply flawed, and one that isn't even applied equally in this book.