For example, I am reading 'The Game' by Neil Strauss which is about the time he was a pick-up artist and about the whole pick-up artist scene/environment. I post something about this book on Twitter, and lo and behold, I get a new follower which is the 'PUA app for iPhone' (PUA = pick-up artist).
Another example: I post something about fitness ball exercises and two seconds later I have several followers who are US fitness trainers.
This really bugs me. Yes, I know I can block them but that's not the point. The point is that Twitter is a space for sharing your thoughts, it is not a commercial market place where others can stalk people around and hawk their wares. Go away!