I twit from time to time, and I've seen this first-hand: pornographic bots made up a large chunk of the Twitter universe. I block a couple a day, and half the time they're already deleted accounts. The link above is Business Weekly, who is concerned that the porn spam will damage Twitter's business model, which, at the moment, seems to be to give away a lot for free without asking for any money in return - which, on the outside, looks like an iron-clad business model immune to the recession or unemployment figures, but, well, spammers can fuck just about anything up if given enough time.