I thought this was an interesting and helpful blog post from Stephen Altrogge (The Blazing Center)
Here are a couple of quotes:
We were made for real, person-to-person relationships. Superpoking, chatting, and throwing snowballs at each other doesn’t constitute biblical fellowship. God created us to “bear one another’s burdens” (Gal. 6:2), “meet together” (Heb. 10:25), “rejoice with those who rejoice”, and “weep with those who weep” (Rom. 12:15) This doesn’t usually happen in a meaningful way on Facebook.
Just like television, music, and movies, Facebook is a loaded gun when it comes to temptation. Flirting, sexually immoral applications, sensual sidebar ads, inappropriate pics uploaded by friends – it’s all possible. Can it be avoided? Sure. But the fact that it exists should put us on guard every time we log in.
Stephen also lists a couple of links to some similar articles: