My crush giving me mixed signals. What should I do?

[Dont Eat My Jello!]

From what I can tell, when you both started talking, she was quite shy. That's why she would look down a lot. But since she doesn't do that any more, it means she's comfortable around you. And that's a plus!
Also it's great the two of you have a lot in common, and things you like to talk about.
And the fact that she still laughs/giggles when you stumble or say something dumb, shows that she doesn't think you're like a moron or anything. It also shows she likes having you around, and you can make her laugh. That is another plus.
It's also great she stood up for ya! And this obviously shows she loves your company, and doesn't give a crap of what others think.

But here's the thing, just because she doesn't respond to a tweet or what-ever, doesn't mean she doesn't like you. I think you're putting way too much thought into that.
The way she talks to you, how she tells you to get a twitter account so you guys can talk more.
Well, this girl really likes ya.

I think it's very possible she likes you more than a friend.
I know this may sound daring, but how about say to her one day, "You're really pretty". Obviously you don't have to say it JUST like that. I know, it kinda sounded creepy. haha
Do it more in a, "Wow, I just realized something..."
When she talks, make eye contact (which I think you're already doing well). Like stare right into her eyes, and show that you really get what she's saying. Don't pretend of course, but just let her know you care as well.
You can also try casually holding her hand, and maybe ask her to go to a game with you, or see a movie. You could even rent one, and have her over. Eat popcorn, play video games, and just maybe go for a walk and talk. The little things you do together can really help you get your feeling across.

Anyway, I hope I helped! Sorry if I didn't. I tried.
Good luck though! (srry this was long, heheh)