This girl is mutual friends with my friends. We've hung out a few times.

Last time we hung out, we had a good time, but I was kind of a pussy and didn't take things outside of a friendly comfort zone.

She added me on facebook and I messaged her. We chatted for a bit, mostly joking about the prior night. Then I suggested we should swap numbers. She replied and asked for mine, I gave it.

After this she didn't reply for 2 weeks, and I kinda forgot about her. A few days ago she texted me out of the blue (note: I know this is her since Ive seen her number before):
her: "Hey Loser"
me: "Lol julie is that you?" (this isnt her name, just a random girls name I made up to make it seem like I was expecting a text from someone else, lol)
her: "yes its julie"
me: "nope, i think youre lying!"
her: "do you kno who this is?"
me: "no who?"

This was 2 days ago and still no reply. Around this time she also replied to my 2 week old message on facebook. Whats going on here? Is she just playing hard to get or is she not interested?

You should have just played along, and made up some stories, like "Btw, I had a great time last time, how was the breakfast i made for you?" until she called it out herself that shes not Julie.

Anyway, I dont think she's playing hard to get, she simply just isn't interested enough to jump you. She was probably busy and forgot to reply.
Girls dont do 2-weeks-playing hard to get, they know they will lose the guys interest fast that way. They tend to keep you on the hook, in order to do that, they will feed you little attention occasionally.

She's interested enough to add you and exchange number, however it doesnt mean she sees you as potential mating stud, that probably explains the whole thing, either you need to escalate or you gonna stray down the friendzone path pretty soon.

Haha - I actually was considering replying with "you left your -article of clothing- here" to keep it going. I agree with your opinion that she doesnt seem that interested, oh well.

Any advice for best way to proceed from here? I was thinking of laying off, not replying to her fb msg, and not txting her again and just waiting until we see eachother again through our friends.