In some cases I think you should call the cops if someone is "following" you. However, unless you can prove a history of stalking or something, its not "illegal" to follow someone.
Furthermore, if the police are called about someone who is "following" them, I don't even think thats grounds for a Terry stop. The person isn't committing a criminal act. At the most the cop could go ask the individual who's doing the following what they are doing, however they wouldn't have to answer that question, as they are free to go and have 5th amendment rights.
People call the police all the time informing 911 that someone is taking pictures of them, their children, their property, their car, I was being followed, etc. Sometimes, police do get a call from the "follower" stating that he is a license private investigator, neighborhood watch, etc.... and informs dispatch what is going on. In the case of the private investigator, its often the target individuals spouse who hired them. Imagine that.