Why is Hitler, rather than Stalin, the default Internet insult?

A local high school had some minor online vandalism yesterday, as we reported (calling it a "prank" rather than fussing about hate speech or finding one person to criticize it so we could put "community outrage" in the headline, which would give it way too much gravitas).

The prank involved putting a picture of Hitler on a Facebook page that looked like the school's official page - it was actually a "community" page, which can be created by almost anybody. Somebody also dumped the Hitler picture on the school's Wikipedia page around midnight (labelling it "school mascot"). The Wikipedia change was undone almost immediately, and Facebook removed the Hitler picture within a few hours of our reporting on it.

This led me to ponder something, however: Why is it always Hitler? Why not Stalin?

Josef Stalin who was just as horrible a human and caused the deaths of at least as many people as Hitler, and the gulag system was as mind-numbingly, deliberately awful as the Holocaust. So why doesn't Godwin's Law , which says that any Internet conversation invariably degrades into comparisons with Hitler, also cover Stalin?

There are other awful folks in history which could be cited, of course: Pol Pot, Idi Amin, Caligula, but they're not as well known or are historically distant. And then there's Mao, who cause many many millions to die horrible deaths; but perhaps a comparison with Mao might also be misread as some sort of comment about modern China and politics.

But I can't think of any argument that makes Stalin less suitable as a metaphor for "as bad as humanity can be" than Hitler.