In the last several days I've noticed a large increase in the rate of bots following me on twitter. It went from perhaps one a day to dozens. The bots are following 100-200 people, and have always sent zero tweets. They sometimes have an avatar picture, and their bio always sounds convincing. The avatars and bios are probably harvested from real Twitter accounts, and sometimes they get the default avatar. They appear to avoid any bios with a link.
The email from Twitter tells you what client was used to follow you. They have cycled through various third party clients, and then Twitter's mobile web page. Most recently the email contains no mention of the client, and I don't know what that means.
What is interesting is that this influx of follow bots never appear to send any tweets. An entirely different herd of bots has started spamming via @-replies, with a link purporting to offer a free iPhone. The bots which send the spam follow zero people.
I wonder: are the botnets now fielding offensive and defensive teams? By this I mean using the offense to send spam, and watch for twitter users which block them. They can check whether the bot can still see the tweets of the users it has spammed. Users who react by blocking are likely reporting the bot for spam, and can themselves be targeted by the defense. The defense has never sent any spam, and can report legitimate users to try to get their account suspended.