Spam coming to twitter
I guess one of the signs that your web service is taking off is that spammers are targeting you. In the last few days more and more fictitious followers have surfaced, obviously for the purpose of sending twitter spam once you follow the person who is following you (as most people do almost without thinking).
Update: Definitely not automatically. The last batch of spam followers are still active accounts. Or maybe they figured twitter’s threshold and they are avoiding the automatic suspension.