A focus on the mediums through which these shooters express their views misses the forest for the trees, which is that this is fundamentally a political problem.
There is an ascendant political ideology in the United States, and in the West more broadly, that is rooted in a fear that White supremacy is being eroded by immigration and demographic change, and that violence against minority populations is an acceptable response to these changes.
This ideology is promoted by the President and prominent members of the Republican Party and the party-affiliated media; government goon squads are used to harass and ethnically cleanse those populations; and "lone-wolf" paramilitaries terrorize them directly, while allowing their "respectable" enablers and ideological supporters some degree of plausible deniability.
The solution to this problem doesn't lie in technology or communication platforms or mental health awareness or anything like that. It lies in organizing politically to defeat the supporters of this ideology and take power away from its proponents.