To have done so DURING the election would have created accusations of bias. This was always going to be a big story post-election. Stories about troll factories and the Macedonian clickbait troll town were out before the election. The comment trolling phenomenon hasn't even picked up much coverage yet.
That said, a lot of blame does need to be apportioned to western intelligence agencies. The idea that hostile foreign actors could (if they wanted) manipulate large segments of electorates seems to have escaped them entirely. This is especially damning since they saw how those tools were effectively deployed in the Arab Spring and elsewhere to foment revolutions (if they weren't behind those movements themselves). It is an epic fail, and joins a long list of intelligence failures.
Another place where Facebook's failure to detect fakeness has proved costly is in their social graph.
A lot of sites moved away from comments platforms like Disqus to Facebook in the hope that the quality of discourse would improve and trolling would decrease. Instead, clicking on some of the most vehement commentators' names would invariably lead to suspiciously bare accounts (and often suspiciously fake names) with a handful of friends themselves, all with similar characteristics. Unfortunately the people who are influenced by this sort of thing are not tech savvy enough to do even this basic level of checking.
There is a a sort of "uncanny valley" that a technically savvy and experienced person can detect when looking at a fake profile, that I daresay Facebook's algorithms just can't.
Then there is also the problem that Facebook really doesn't care about blatantly fake accounts until they are reported. The sorts of people who are trapped in some filter bubbles are unlikely to be savvy enough to know how to report these profiles (I've reported many-dozens at least-ranging from community noticeboards to cupcake businesses, all pretending to be people and missed by FB's much vaunted ML)
Of course, there is a certain irony in using a throwaway account to discuss fake accounts, but without a patina of "realness", it triggers a greater level of skepticism, which is a good thing.
When the history of 2016 is written, a large part will be filter bubbles and trolls expertly manipulating huge swathes of electorates enabled by the hubris and greed of Social Media networks.
That said, a lot of blame does need to be apportioned to western intelligence agencies. The idea that hostile foreign actors could (if they wanted) manipulate large segments of electorates seems to have escaped them entirely. This is especially damning since they saw how those tools were effectively deployed in the Arab Spring and elsewhere to foment revolutions (if they weren't behind those movements themselves). It is an epic fail, and joins a long list of intelligence failures.