On Sunday, Michelle Goldberg talks about this problem for female writers with feminist messages, "An unbearable burden: Feminist writers are so besieged by online abuse that some have begun to retire", link here. She talks particularly about writers published on xoJane. It seems that some men resent the idea that a female would not allow herself to be "available" to him, and mentality that is said to live on some campuses. It also lives in undeveloped parts of the world, particularly with radical Islam.
The "Dummies" books has an advice page on how to detect "spam" followers in Twitter, link here. I see this all the time, a party that looks irrelevant to my content follows me and drops me in about two days, as I don't follow them back. Some sites generate messages all the time. "Chess Quotes" generates left wing messages about the abusive bourgeois class and some silly but harmless "heterosexual" pictures, but sometime actually sends legitimate news stories, so I left it up! "Healthy Living" generated a lot of aphorisms that actually made sense, but then disappeared.
But the fact that spam-following happens a lot would also denigrate the idea that people should have minimum-follower counts, as that encourages and even rewards spam.