A person who blatantly flaunts their Christianity on Facebook or other social media, but does not pursue their religion actively in the real world or leads the opposite lifestyle in real life.
Veronica posts "99% of people are too embarrassed to embrace Christ as their personal savior on FB.. most won't repost their love of Jesus" then engages in premarital sex, drugs, and generally un-Christian behavior in real life.
Me: "Wow, what a Facebook Christian."
Refers to "Christians" who will "Like" any picture of Jesus, because it involves almost no thought and even less effort, but who post arguments against Bible verses or in support of things that go against Biblical principles. In essence, a Facebook Christian is like a Lip-service Christian, except even worse, because a Lip-service Christian at least knows Bible verses and can talk a good game, whereas a Facebook Christian only knows how to "Like" pictures of caucasian, hippie Jesus posted by such pages as 1-800-Smak-dat-hoe and FML Daily.
Liking pictures of Jesus only makes you a Facebook Christian; you have to actually believe what the Bible says to be a Christian.