Social skills is a broad and everchanging aspect of child development. Children mature at different rates and have spurts of growth (intellectually, emotionally and physically). I would consider other interventions than retention in school for social skills. Retention has a lifelong impact on children and can be devastating. If your child is behind academically, interventions to remediate the academic issues are recommended.
This summer enroll him in a social group or activity at church or in the community to give your child more exposure to his/her peers.
Interesting question, but I'm uncertain if you mean that the child has already started school and you/teacher are considering retention as a solution to "catching" your child up with his/her social abilities? OR are you talking about a child that has not started school yet and feel that another year would benefit your child by maturing to a place that would be a better benefit to them by waiting another year before starting?
If it is the later, then absolutely, keep your child home another year to allow him/her to develop in maturity and they will perform much better once they do begin school. A respectable Kindergarten teacher had told me that she had never had a parent to regret holding their child out another year, but had seen many to regret their decision to start a child. So, what you really want here, is no regrets.
Otherwise, if your child has already started school and you or the teacher feel that your child is not performing on a social level as the other children, that is different and as long as the child is performing academically, I would not want to hold the child back. It would be unjust to have a child repeat subject matter that they have already mastered just because they may be shy or quiet.
Personally, I think it would be devastating to a child to be retained for their social abilities and heart breaking to think that their parents/teacher, etc. doesn't feel that they are "up to par". We send our children to school to get the best education possible not to be social.