Need real advice on what to do regarding a woman I have feelings for

I have a huge crush on this girl that works across the hall from me (we're in grad school).

A couple months back I was messaging with her on facebook and at one point I said "we should ditch work sometime and get a drink". She replied "sounds cool". After that I stopped talking to her for a month or so (I was busy) until recently, because I kept thinking/dreaming about her frequently (not sexual dreams).

Anyways, I have been chatting with her on facebook messenger over the course of the past few weeks. At least once every other day we will be messaging off and on over the course the day. I have always initiated these conversations, but she always replies. I will send her jokes, funny pictures, we will talk about work and make fun of our bosses, etc.
My only concern is - she never initiates the conversations.

In any case, I'm crazy about her, but its driving me crazy because I don't know what's on her mind and I don't want to ruin my chances by being to bold too quickly causing her to raise her defenses. She is a shy and introverted girl, I can always make her laugh, and when I pass her in the hall, she always glances at me then looks down to the floor. She seems more closed down in person most of the time in comparison to who I talk to online.

Anyways, I want to post here the last message exchange we had on facebook, which was Thursday night. I initiated this conversation, but also ended it first because I had to work late.

(10:00 PM Thursday night)

ME: Lmao. That movie is hilarious.
HER: Lol yeah
ME: I guess I will go get to work and make the annoying boss happy. I'm sure he will call here later.
ME: Night, Pamela.
HER: Working late tonight?
HER: Good luck with the annoying boss
HER: Gnight!!

(6 hours later at 4:00AM Thursday morning I responded)

ME: Yeah. Just finishing up.
ME: We don't have to work overnight - I just get more work done if I do.
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She saw this last message from me early yesterday morning, but she didn't respond. Since I sent that message early Thursday morning, I have not messaged her again. Should I hold off from messaging her and wait to see if she will say something?
I just don't know what to do. I'm wondering - if I back off for a bit - would this be an effective test to see whether she likes me back? I know she is an introverted girl so one part of me thinks she might want ME to do all of the initiating.
What do you think I should do? All thoughts are welcome- I'm all ears. Thanks Everyone. God Bless.


P.S. We are both in our late 20's ....if that gives some context.