The way I've heard it is this: People tend to use the same sorts of social media as the gender they were assigned at birth. Or, to put it another way, they don't change social networks when they discover themselves and/or transition. Reddit and its descendants Lemmy and Kbin are dominated by AMAB users, and therefore see more transfem users than transmasc users. I'm assured there are a lot more transmasc people on Tumblr.