Let me paint a picture for you...
A nice family, married 9 years. 3 Children. Perfect attendance at church. Wife is in the womens bible study, husband participates in the worship band. Kids can recite versus from memory.
Now, who are their friends?
I run into a lot of Christians, Christian Couples and Christian families the problem is they only associate with other Christians. Many say they are "protecting their family" or "why would we want to associate with non-Christians?" 
I believe we need to associate with Christians and non-Christians alike.
I run into a lot of well meaning Godly people but they cannot associate with non-Christians or they simply can't relate with some of the struggles of non-Christians.
I believe in being "the light of the world" which means you have to live and associate "in" the world but not necessarily "of" the world.
As I have talked about before with parenting, I believe many Christians today try to create a Christian Cocoon in which they try to protect their children and family from any "outside, non-Christian elements."
We have many Christian friends but we also have many non-Christian friends and co-workers and the funny thing is, we don't minister to many of our Christian friends but we do a whole lot of ministering to our non-Christian friends.
Many of these people were raised in a Christian church but for one reason or another stop attending or left.
Now please understand their is a subtle way to do this as well. You don't want to be the Christian that constantly brow beats others and makes people run when they see you coming. We don't ram Christianity down their throats but we also don't hide our own beliefs and we often speak of our faith and how it has changed our lives.
I truly believe that only associating with Christians is selling ourselves short of many ministry opportunities.
Case in Point:
A co-worker was having problems with his marriage, he always heard me speak highly of my wife and of our marriage and heard others say I had a great marriage. One day he asks me to go out for a drink at a bar after work. (Many Christians would object simply for the fact of having a drink at a bar.)
I could see he wanted someone to talk to so I said yes. He laid it all out for me over a couple of beers and then asked how I did it. I then gave him a summarized version of my testimony and told him that I often struggle in many areas of my life including marriage but my faith has helped me through and given me further guidance in all areas of my life.
Why did he come to the Christian for advice when he was an atheist?
Why did he feel comfortable coming to me instead of some of our atheist co-workers?
I'd like to think it was my "light" that attracted him but I really believe it was God's plan.
Did he come to Christ there and the angels in heaven rejoiced? No.
But the seed was planted.
As far as hanging out with non-Christians, drinking in a bar giving marital advice, I'll let the bible speak and teach the final lesson on this.
The Pharisees said of Jesus: "The Son of Man came eating and drinking, and you say, 'Here is aglutton and a drunkard, a friend of tax collectors and "sinners."
--Luke 7:34
Jesus taught and associated with sinners and tax collectors. He drank wine and ate at the houses of some of the most notorious characters of his day.
I let him be my example.