There's definitely bots on Twitter and Reddit. There are novelty accounts that are openly bots that can respond to people. I have no doubt that bots that are supposedly real people exist. The problem is that the characteristics of bot posts - posts consisting of set phrases rearranged in various ways, only responding with set phrases instead of addressing the post, ungrammatical or incoherent posting - also apply to real people. It can be tough to tell real humans from bots. But on the other hand, it's also cheap to pay third worlders to spread your message.
Bots pretending to be real women are used on dating sites to get men to buy memberships. The Ashley Madison leaks showed that at least 70,000 accounts were fake. These accounts could respond to messages from male users, encouraging them to purchase services. Some analysis suggested that the majority of female accounts were fake.
There was a history of bots posting on Usenet going back to the early 80s. As garakfan69 mentioned,
Mark V. Shaney was a Markov chain program based on posts on and posting to net.singles created by employees of Bell Labs. Many people thought that Mark was a real, if odd, person. A Markov chain, to simplify, analyzes a corpus of text and creates a statistical model weighing the likelihood of certain words or phrases following another. This model can be used to generate new texts which are largely grammatical, and can be seemly meaningful for short texts, but large texts are disjointed or incoherent. Markov chains are still used, notably for Twitter *ebooks accounts.
Another famous one was a spambot called
Serdar Argic, the bot would scan all of Usenet for mentions of Turkey or Armenia and post messages arguing that the Armenian genocide never happened and that actually it was the Armenians that had committed genocide against Turkey. The bot famously would respond to posts about the animal or food turkey.
Several years ago I decided to check out Usenet. Outside of piracy groups and a handful of legit boomers still posting in certain nerd groups, it was like a ghost city. I saw dozens and dozens of discussion groups filled with nothing but spambots. Months and months of posts from multiple spambots posting every day. I found it strange. Spam killed Usenet, and long after any actual posters had left, these bots were still there, posting spam that almost no human would actually see.