I have stalked girls who I've just met and had a bit of chemistry with, and am wondering whether to go out with them or not, thats a bit of facebook research really. In one case, I actually went off a girl who I had liked in person, because she seemed to post too many statuses on facebook, and had made 'notes' of all of those "answer 20 questions about yourself". I decided from that that she wasn't cool enough for me.
However if it's somebody I'm interested in, that isn't interested in me back, I will keep her on facebook but I block her statuses/photo updates etc from coming up on my news feed.
You just see things that make you feel jealous, even if it's fairly innocuous, eg seeing the girl in photos with a good looking guy, or seeing some new guy she has befriended starting posting on her wall with winks at the end of the message etc, and wondering if something is going on between them.
Then if she does go from being single to 'in a relationship' but doesn't say who with, you wonder is it for real, is it a joke, who is it with? Then her female friends all 'like' the status and write stuff like "oooh am I missin somethin? do tell xxx". You just think DIE DIE DIE.
The problem with facebook stalking is people think that its fun and think they are enjoying the power of being able to surreptitiously snoop on the object of their desire's profile, in reality you aren't seeing anything that they haven't allowed to be publicly viewable anyway, and if its stalking based on fancying someone who you're never going to go out with, then its just torturing yourself. It makes the process of getting over somebody a lot longer and more drawn out if you are stalking them, and you risk falling into relapse if you've got over them, and then 6 months later, their status goes to in a relationship and it makes you feel bad again.