Re: In the short run
> People have to be paid, and have limited bandwidth for manually posting stuff
Paid? No, all you have to do is find the right hotheads who will go willingly (far) beyond the call of duty. Fanatics aren't there for the money.
As about bandwidth, I fail to see the point. First of all being an AI won't magically improve your connection, second you don't need much bandwidth to post toxic stuff to the half dozen popular social networks and maybe a blog or ten. Yes, you're thinking spam, but that's hardly the most efficient (or even a good) way for propaganda campaigns. Besides the negative initial impression (nobody likes being spammed), most spam gets automatically deleted, while social media posts remain there for all to see, reference, and re-post. Your eager and willing fifth columnist will certainly already have an Internet connection and social media account of his own, so he's immediately operational with no expenses on your part.
Concerning the amount of fake news, why rely on one single AI when you can have hundreds of eager fanatics lovingly disseminating your propaganda all over the place? The added benefit is rock-solid resilience: You can't be shut down, because there is no single point of failure. Just keep recruiting new soldiers, they are out there just waiting for the call, and are all utterly dispensable!