Russians were putting out. It wasn't all pro-Trump, or anti-Hillary either. Some was simply to inflame touchy issues (Black Lives Matter, etc). Some of the political rallies were also driven by Russian efforts. On balance, the propaganda was in furtherance of Trump's election.

For as much grief as social media companies catch, they've been working to uncover this kind of fraud. It's just a lot easier to create ad hoc accounts than it is to monitor billions of users, and sift through content. The linked article shows how this is an ongoing issue, and not something easily fixed.

A good lesson to take away is that deceptive messages only have power when the person reading it is receptive / uncritical. When you read something that makes you angry at some other person or group, stop and think about it for a minute. Seek out their point of view, and maybe give a little more benefit of the doubt initially rather than retweeting / spreading the content with an expletive attached.