I've been on Facebook for a few years now. In that time, I've learned about a few things that work well and don't work well for Christians on Facebook. Here is my advice:
1. Focus on honoring God. This is the most important advice I could possibly give to a disciple of Jesus Christ. Share a scripture or inspirational thought every once in a while. Share an encouraging word about what God has done, how he has answered prayer, or how faith in him has helped you.
2. Focus on others. Don't post too often about yourself or your life. Concentrate on demonstrating to others that you care about them. Let them know that you are praying for them when they are facing difficult situations. Take the time to "like" their photographs and status updates. Leave an encouraging comment when someone shares something good and wholesome. People don't post on Facebook in order to be ignored.
3. Avoid partisan politics. Your mission field includes Democrats, Republicans, and independents. You can affirm the value of life, the importance of marriage, the need to care for the poor, and the virtues of sexual purity without mentioning politics. You can be more effective when you avoid unnecessarily offending someone.
4. Avoid theological or religious discussion groups. You can get bogged down in useless arguments with "experts" when you join such groups. It can be more than a waste of time. It can be counterproductive.
5. Be careful with using humor. Sarcasm is especially dangerous. It can be easily misunderstood and unnecessarily offensive. Certainly humor can be used, but make it lighthearted and fun.
"Live wisely among those who are not believers, and make the most of every opportunity. Let your conversation be gracious and attractive so that you will have the right response for everyone" (Colossians 4:5-6, NLT).