its the kind of coworker that I have casual conversation with, not the kind you can joke around with type of closeness if you get my drift, but she seems kind of cool, would it be weird to add this coworker?
I'm the type who uses online stuff like facebook for close friends. You don't want to put something on there that may offend them. Plus, you don't need coworkers knowing your personal business.
I add coworkers all the time...what you really should do is change your privacy settings and create a group specifically for coworkers, which you can keep them from seeing personal info such as photos and your address/phone #, and more. I always add my not-so-close friends under a more limited setting just because it's basically none of their business