Facebook can be either good or bad, like money can be good or bad. It depends upon the user, and their heart.
I struggled for months, and finally joined Facebook to satisfy a sister who wanted to stay in touch, and who is a stay-at-home Mom and has the time to be on the computer. She has regained contact with cousins, and other family members because she has Facebook.
Once I joined, I realized that I can use this medium as a tool.
There are drawbacks, and things to be cautious of, as with anything else on the Internet.
It is (according to many in the conspiracy theory camp) a tool used for spying by the CIA, and other government organizations. Facebook can keep all your data you upload or put on your site, forever, even after you close your account, or whether or not you have deleted the info.
Consider it "publishing," without any form of copyright, every time you post something. It is available to the world immediately, and to the government forever, even if you delete, or shut down your account.
If you are ok with that, then go ahead, and use it for God's glory. Like anything else, you can get addicted, distracted by it, or waylaid by it.
So you need discipline. But you do with anything else as well. I respect Paul Washer, but his audience would be greatly increased if he maximized the Internet. Others are, and so his messages are getting out to people he could never reach without the full impact of the Internet.
If used as a tool, Facebook can have a wide impact for God.