i honestly doubt these forums make a large percentage of the bots, I could definitely see Facebook being assaulted like Normandy but I don't use social networks so I'm not on their radar.
You don't need a large percentage on any particular site. One troll (per shift right, not like there has to be only one person using it) can easily handle a few accounts on any particular website. This website in particular has about 400-1500+ people surfing it (according to the main page) at all times.
Trump gave the Russian military the data from the RNC, and through hacking businesses like credit reporting companies they have more than enough data to know enough to start a data file on everyone that is just going to keep growing with every log in online. And that gives them a personality profile on you, which links to your family and friends (and if any of them have Facebook a live feed into basically everything about them and who they talk to on their cell phones) and only helps the power of the propaganda attack.
https://www.rollitup.org/t/ap-cyborgs-trolls-and-bots-a-guide-to-online-misinformation.1005699/post-15310450
Websites like this are the way to spread links to the nonsense and increase the notability of propaganda websites and if someone who is an actual 'influencer' (with spyware and trolls following them everywhere like a std) does log in here, the troll spams the forum with whatever flavor of crazy to get everyone worked up and attacks the actual person when they say something similar.
Between attacking kids on their video games and hiring actual writers to push their propaganda,it is a lot safer to just understand everyone is under constantly evolving trolling attack anywhere they happen to try to have a conversation, even little old grandma's visiting their 'friends' knitting website.