Should I be worried that she didn't message me back?

[James]

I did some little flirting with her on facebook for like a week (I liked her new profile picture, we had a poke war, commenting random google images on her wall for like an hour lol, etc) and then Monday she poked me, and I poked her back, but now she hasn't poked me again since the (she's usually almost never on during the school week though, last week was weird). I wanted to ask her to hang out this weekend sometime, so then two days ago (Thursday) I saw her on and I said "hey loser" to her on chat, but she didn't reply and it turns out she didn't even see it until last night (Friday). Should I worry that she didn't message me back? I mean I wouldn't message someone back a day late if they just said "hey," so I don't think so. And should I try messaging her again tonight or should I wait a day or two longer?
Thanks
PS I know that just because I flirted a bit with her doesn't make her mine for the taking, but it just seemed weird that she abruptly stopped all contact. And I know that this should be a pretty insignificant thing to worry about, but I just really like her, I guess.

Oh yeah and a few weeks ago something similar happened where I sent her something and she didn't reply and then that time she replied 5 days later. Although that actually had something to say, it wasn't just "hey" (I sent her a picture of a turtle and told her it looked like her...she seemed to think it was really funny. That was the first time I really flirted with her.)

[Best]

Don't even trip dude, it's not she's your girlfriend or anything right?
Maybe she's just been busy, or something. Point been if. She's just like a friend don't trip about it to much, try having your girlfriend not text you since last night >.< it f****n sucks

[Amanda]

If this has happened before, i wouldnt worry too much about, esp. it if it has only been a day or two. she might just not know what to say at the moment she saw your message.... maybe wait until you see her in person and talk with her then