Did pretending like I didn't care backfire on me?

It really depends on the guy. Some guys will continue to do whatever they have to, to get your attention, but some will just give up and log off. If I was talking to someone who acted like they didn't care, then I would just leave without saying goodbye.
But anyways, what are you trying to get out of him? Do you want him to chase you? Do you want more attention? Why do you "pretend" you don't care?
I'm assuming you think guys want a challenge and by "ignoring" him, he will chase you more. Well... guys (and girls) do like challenges. It makes them want to work harder. But if they don't accomplish anything, they eventually give up. People like to feel like they've accomplished something. It makes them want to continue doing it. It could be something like, making you laugh, or making you smile when you're feeling down. Something as little as that, could make a guy feel some kind of accomplishment, and they will want to do it again.

If you don't want to give too much, then don't. But it doesn't mean acting like you don't care, or ignoring him. When he's online, chat with him, but not too long. Chat with him for a few minutes, then politely say you have to go because you have other plans. Or because you are busy. Don't be surprised if he calls you right after, to ask when he can talk to you next, or if he asks what you're so busy with...
Remember the less you give, the more they want.
So keep yourself busy.
But you shouldn't have to pretend. Make real plans. Even if it's just laundry. Keep yourself busy... keeping yourself busy will help you not focus so much on him. Because when you focus so much on someone you tend to give too much... and that is not what you want. So, make him miss you. Start planning stuff. But don't ignore him, and definitely don't act like you don't care.