I'm not sure if you're arguing that the overlap was less than I describe or that it was negligible. But I would soften my prior claim far enough to say that, while each site had its own constellation of cultural tropes and styles with which the median user of that site would primarily identify, a large minority or more of each site's most prolific cultural transmitters was broadly and continuously informed by participation in both cultures, such that those cultures tended over time to express increasingly similar behaviors, utterances, and semiosis, despite over the same span each growing overtly more hateful and contemptuous of the other.

(A Freudian might reduce this to superego vs. id, a Jungian to animus and anima. I'm not any of those kinds of mendicant, and make no assertion as to etiology. But it is anything but controversial to suggest we have recently changed our environment in ways that can be unexpectedly dangerous for the young and others whose personalities are incompletely or imperfectly developed.)