If your profile is not set to private then, theoretically, you are posting to the whole world.
You can gain more followers by doing the following:
1. posting quality tweets
2. marketing yourself
It does take time. I am on the brink of 400 followers and I've been a Twitter member for well over two years; having a popular blog *really* helps.
Additionally, you can reach more people by linking your Facebook, blog, website, et cetera to your Twitter account.
If you follow me, I'll help you out and follow you. :-).
http://twitter.com/briealeida