She might be hitting on you or she might just like a little male attention. She might be really friendly or she might just not really understand boundaries, etc... Most of the time if a woman has gone out of her way to contact you through more than one medium. (she called and friend-requested you on facebook. She could have sent you a FB message saying thanks for working with her) she's got some kind of crush/feelings. Doesn't necessarily mean she's going to try to push it any further but you never know.

You're always going to have other people trying to swoop in on your marriage from both sides. How you handle it is up to you. It's nice to get that attention from other women but you really have to ask yourself how you'd feel if your wife was getting that same type of attention. Think about it, guys are WAY more forward and thirsty than women in general so that little bit of flirting you're getting from this girl, your wife gets probably 10x that on a daily basis with a few subway bump-and-grinds thrown in for good measure. If she's not getting gassed off it and adding new FB friends on the regular, neither should you.