Either don't reply or actually SAY something

Right, picture this. You're seeing a girl and things are going pretty well. You're not much of a texter but you'll do it for her. You haven't spoken for three days so she texts you to see how you are and you start trying to have a conversation.

So, a conversation is a two way thing. It includes both people being interested in it, with questions and answers most of the time. If someone asks you a question it's polite to give an answer and maybe reply with a question of your own. Y'know, just so you can show that you're actually interested in the conversation, and not just replying 'yes' or 'no'.

Here's a "conversation" a friend of mine had with a girl.

Jamie: Ahhh tht sucks, u lookin after them again then? i bet uv been a good girl :) wat u up 2 now then? x
The girls reply: i left them x

Now I'm not surprised at his reply because I'm sure I would have gone just as mental he did, if not more.

Jamie: What the fuck is tht? how am i spos to reply to tht shit? just answer the simple questions, stupid women'

Now fine if they don't wanna talk. That's absolutely fine. If you get a message such as the one she's sent to him you're gonna take it as a hint that she doesn't want to talk to you / doesn't like you or whatever.
However, if she texts you the next day asking why you didn't reply, now that's just gotta make you mad. You can't just sit down and take that. Something needs to be done and there's no way I could ever stay with a girl like that for longer than a week without wanting to kill her.