I remember when I first moved to the Bay Area in 2009, I met a smart guy at a startup meetup, a serial entrepreneur with a previous exit. I asked him what he was working on, and he said "Oh, I've been writing a Twitter bot that will follow people of your choosing, engage them in simple conversation, and retweet their tweets. It's building a network of followers - I don't know yet what I'd do with that, but it's an asset that's likely valuable to somebody." I ran into him again a few weeks later, and he'd sold the company.
I think I'd heard from either him or someone here on Hacker News, around that time, that 75% of Twitter traffic was bots and automated accounts. Note that that was early 2009, before Twitter went mainstream and when they still had a really easy-to-use developer API.