How do you get the Twitter and Facebook icons in your apps

I created a Twitter and Facebook SHARE buttons. However, I don't know if i'm allowed to use the official Twitter bird logo and the official Facebook F logo as a custom image button on my app. Are developers allowed to do that or are we supposed to get permission from Facebook and Twitter? Or should I just create a custom button that says Facebook, Twitter on it? I've seen only official facebook and twitter logos on game apps and other apps. How is it done?