Maybe FB propaganda was simply more effective in motivating Rep voting. (Fearmongering about vile brown migrants raping our beautiful white women works better than fearmongering about climate change and the lack of structural health care reform, who knew!?)

Anyway, you are completely right about how overused the Russia line is without data on its interference effectiveness.

Robert Mueller, yesterday: "our investigation found that the Russian government interfered in our election in sweeping and systematic fashion"

So... That did happen.

There is no question it happened -- we know for sure the Russians were pushing hard on this. Most nation-states push propaganda, this was just especially hard and blatant. The question is how much did it help, and were other players, like the NRA, Koch Bros, etc. also involved?

With sanctions freezing huge amount of their assets the Russians didn't really have any serious plays against the US except pushing the soft-power propaganda angle[1]. Essentially they (Russians) just kept trying lots of random things[1], in high volume, until something stuck -- and it worked, spectacularly well too, mostly due to decades of propaganda already shaping the Republican base.

Hilary was marketing hard on all platforms, but some estimates put the Russian spend at $400k USD per month, on top of NRA, GOP, and other heavy Right-Wing spends.

[1] see RAND Corp research about this: https://www.rand.org/pubs/perspectives/PE198.html

How much money was spent in the 2016 election cycle? It was at least $1 billion. $400k/month, even for 24 months, is $9600k, or about $10 million, or 1% of the total spend. It is not by tipping the spending balance that the Russians had however much effect they had.