A school cannot hold a student back, FOR SOCIAL SKILLS! It's these sort of unfair things that get me worked up, even if it's not happening to me! :(.

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.

Hope this helps!

Best Wishes!