Just to note that they may not literally have been “bots”. There are places on the internet where you can go to hire real people to register on sites, follow you on Twitter, like your Facebook page etc at packaged rates. “Get a thousand Instagram followers for $10! Discounted rates!” The followers you get are real people, who are employed by firms to provide this “service”. I know a couple of people who do this and it’s decent pocket money (especially if you’re in a country with a low cost of living) for a few clicks in the day. They register, follow and don’t interact after. So it may not have been “bots”, though for all intents and purposes, they’d behave like one.