I'd say it's good if you have local friends on there, but to meet new Christian friends, I wouldn't recommend it. I agree with you, we should start witnessing on these sites like facebook. We can spread the gospel throughout the world without even lifting a foot. Praise God
Matthew 28:19
"Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost:" KJV