In 2013, I remember people were building Facebook audiences by ripping Facebook ID's from relevant open groups - that was insanely targeted audience, and while it wasn't general practice, it's definitely something advertisers were using. None of what comes from that is unexpected results. Your argument appears to be that Russian authorities with a $100k budget were 1000x better at manipulating Facebook's audience targeting tools than the people that had been paid to do just that for years - is that true?
I do not like Facebook, personally, and I don't like Trump. But this appears an unlikely scenario.
-> I sponsor an ad targeting a certain audience
-> Byron Shifflett comments on the sponsored content in a way that makes it clear I reached him
-> Under another account, I reply to his comment
-> I send a friend request
-> He accepts
-> Now I can reach him just by posting my nonsense on my second account
-> He shares. That's what I like about him.
I saw a lot of nonsense get spewed on Facebook- things that could be quite simply fact-checked, and people completely bought into them. Also, contradictory emotional appeals were spread (without, of course, the responders recognizing the contradictions). Finally, I was on a forum when someone gave stupid numbers concerning Obamacare, that I quickly checked and challenged (he left and never came back).
Having discovered that people by and large won't think critically about what you're saying, and using the viral nature of emotional appeals to create a general sentiment (which will later be rationalized) payed off for somebody. I don't know if it was the Russians or not.