Presumably these bots make up a very small percentage of traffic in comparison to how much they make up in terms of comments.
And I doubt they have visibility on the most popular social networks.
The reason these bots are able to succeed is because the average person comments and votes so little that there's huge volatility between a power user and an average user. Power users are also not that rare, so it's hard to determine who's a power user and who's a bot (though most power users probably are bots).
You can make up a significant portion of comments and votes (especially on a topic) with a VERY small amount of the total viewing traffic.