ARE YOU A STALKER??

I read with disbelief a fb friend have been stalked by someone again. Accordingly to her, this is the third time ,and all different people. She lamented why she has such misfortune. I was tempted to say because of her beauty, but I did not. Because this lastest one happen to be friend and she did not want to make a big issue out of it or report it. I wonder how do people term a person as a stalker? Does a person who is regulary sending you messages or checking on you be consider as one?  Most stalker are obsessed with the person they stalk and usually after a break up. What if there is no relationship involved. Does that count?  Surely there is a different between showing concern and stalking??  At the most, you friend might think you as a nuisance, but surely do not you qualify you as a stalker.  Can someone throw some light on this??