Edit: I'm not saying anything negative about the female side here, just the male one
On one hand, you have fanfics, kpop Stans, fandoms, Sanrio, etc.
On the other you have literally 4chan, literally reddit, circle jerks about muh superior white music genres like metal, modern rock, and being "cultured" by liking suffering porn with socially conscious hip hop, Roman history, WWII history, military obsession, geopolitics obsession, gamer shit with n-words and Japan stuff, some weird shit with obsessing over your body/facial features to be the perfect Eurocentric ideal, etc.
Also shout-outs to pro incest and pedo anime culture
In the words of a friend who agrees: White men just cannot be trusted to be left alone together on the Internet
femcels are nowhere near as large as reddit or 4chan
They were on Reddit and 4chan.
But really they were 'all over' Tumblr when I was there like a decade ago. They certainly aren't as ubiquitous or as famous as the male variety, so probably not ever as large, and certainly not as large as the male ones are now. My guess is most of them are anti-trans now.
Fair enough, the anti-trans stuff is pretty ubiquitous in chronically online female spaces now, so idk if it's 'gotten better' so much as 'changed character'.
Hence the word "dominated" in my statement