I've wondered that as well. I'm not "active" on Twitter but I log on occasionally to see if there are any interesting tweets in my feed. Every time I log on I have a new follower from penny stocks twitter, get rich quick schemes, and various other fake profiles. This seems to stay stable at around 20 fake followers as old ones get erased and new ones follow.
It seems like amateurs are more capable at detecting spam than the entire company but I sometimes wonder if they just know about it leave the spam bots because once they crack down, new ones will just pop up. Or if they keep them around at a tolerable level that doesn't drive real users away but still allows them to publish a higher "user count"
This may also be in part to more active users of Twitter hitting the "report spam" button on those spam bots. If a spambot tweets at me, I'll go do that. I'm sure I'm not the only one, as I never see a spambot with more than a handful of tweets showing up in my mentions.
It seems like amateurs are more capable at detecting spam than the entire company but I sometimes wonder if they just know about it leave the spam bots because once they crack down, new ones will just pop up. Or if they keep them around at a tolerable level that doesn't drive real users away but still allows them to publish a higher "user count"