I'm interested in the HTC ChaCha but I don't like the Facebook button. I'd prefer it to link to my address book or something like that. Is it possible to do that or in the worst case disable it?
Is impossible for you to attempt to make the change withouth the device been rooted.
According to XDA Developers Thread, you need to go into your system folder located in your root, to change the keyboard layout file located here:
Try Facebook Button for HTC ChaCha or its lite version.
It is a normal standard-abiding Android application, which installs from the Market, does not require any hacks, and works on unrooted devices — so it'll keep you warranty safe.
Feel free to check also its support thread on XDA-Developers.