Just play it off casually. You got her added on social media, you had a quick chat. That's a pretty good job, in my opinion. Remember little, casual steps, don't try to rush things or start making bad assumptions. As of right now you have made it to the status of being a friendly coworker who she was willing to add on facebook and chat with for a minute. Again that's pretty good considering you two hardly knew each other prior to that. If she went offline, just shrug it off and close the chat window, go do other things for a while, and be sure to smile and ask how it's going the next time you see her at work.