When is it acceptable to add a girl on Facebook after meeting her a few times?

[Sarcasticscarecrow846]

Ask her the next time you have a good chat at the gym. To make it seem unexpected and not make you look like a stalker, ask her if she had a Facebook. This will be the bait. You already know of course, but if she responds eagerly, then go for the kill. Ask if you can add her. If you do become friends on Facebook, chat her sometime with regards to going to the gym blah blah blah to make it more casual. And once that has been happening for a few weeks, start saying nice things by chat and when you do see her, start saying a few good compliments to impress her. That how things would start to fall into place.

I've stalked a few people on Facebook, some of them I have seen on the same college but nothing serious. Just wanna know more about them so that I can pull out an ace card to impress them if ever our paths should cross. There's another cute girl whose Facebook profile I would take a peek regularly. I don't know her but she did go to the same highschool as me.