As with all pre-mating rituals (both new and tech-based and old and non-tech-based) someone looking you up on a social network is simply part of the game if the person is attractive to you. If they are unattractive to you, it is [e-]stalking.
And for some people, someone looking you up online so that they can "casually" steer an initial conversation to a pre-researched interest is itself a signal that makes that person less attractive to them.
EDIT: I've known otherwise-attractive guys who've done things like that and provoked "creepy" as a reaction; I've known otherwise-attractive girls who've done similar things and provoked "desperate" as a reaction.
Sometimes it works, sometimes it doesn't, but in general the further you get past just talking to mutual friends for "research" the more you're risking coming off bad. The real trick is trying without looking like you're trying. "I dunno, fly casual."
The thing that gets lost in all these attempts to come off good initially is that just being able to talk to someone entirely off-the-cuff and carry a conversation is a very valuable skill—one that's worth practicing!
It can come off as creepy if you know a lot about a person, but that doesn't necessarily just come from FB stalking.
For example I have a friend who has an extremely good long term memory , he can recall the smallest details about the most insignificant events and conversations that happened many many years ago. He also basically carries an encyclopaedia of trivia around in his brain.
He eventually learned that he had to pretend to be dumber and more forgetful to avoid being thought of as creepy and pedantic.
She was teaching (as a grad student) and realized on the first day of class that she already knew the names of almost all of the students in the class, and little bits about them, from random encounters (like "I was sitting behind those two on the bus a few months ago").
She pretended to be learning their names, anyway, because that's just way too creepy, as you said.
Her memory serves her well in life; she writes fiction. She's also really useful for me to have around -- I have a terrible memory for data I'm not actively using (possibly due to lots of general anesthesia as a kid...), so she remembers tons of things for both of us.
Certainly YMMV greatly on these things. Speaking for myself, if someone simply looks up public information on the net about someone else they are interested in, that's pretty normal and anyone who claims to have never done it is likely full of shit (or very old).
The creepiness comes in depending upon how they use that information. If they use it to lie and suggest their interests are aligned in ways they aren't or in other deceptive ways then yeah that's creepy but in a way that is actually independent of the so-called "e-stalking". It would still be creepy if they learned the information they used from a friend or whatever other old-school method.
If, on the other hand, the person sees on Facebook that you're a huge fan of some hobby and they too are actually a huge fan of some hobby and they mention that, I don't see that as being creepy at all, but again YMMV.
