I'm thankful you are concerned for your old friends; no one knows better than you do how empty their lives are, and how much they need Christ. The Bible says, "Why spend money on what is not bread, and your labor on what does not satisfy? ... Seek the Lord while he may be found" (Isaiah 55:2,6).
This doesn't mean it was wrong for you to leave your old friends behind when you came to Christ. Friends can put great pressure on us, and they probably would have tried to make you return to your old ways. In other words, you probably needed time to grow in your faith and establish new friendships with Christians who could help and encourage you spiritually. The Bible says, "He who walks with the wise grows wise, but a companion of fools suffers harm" (Proverbs 13:20).
Pray for your old friends (and have your Christian friends pray for them also). At the same time, don't allow yourself to return to the things they like to do. Renewing a friendship with only one person may be wiser than allowing yourself to become part of an entire group.
Above all, focus on Christ, and ask Him to keep you strong and help you be a witness to His forgiveness and love. Your friends need Christ, and God can use you to help them.