I believe that the socialization concerns most homeschooling parents have about public schools are quite valid and easily observed, so I am providing a few opinions from others that I have found. Please feel free to add your own.
Public School vs. Homeschool Socialization by Brenda Hoffman
Many public school parents often ask homeschool parents, "If children are taught at home, won't they miss the valuable socialization that takes place in school?" Truth be told, most homeschool parents feel that the public school's social life is enough of a reason for them to homeschool....
Public schools restrict these children to a world which adults believed children wanted. This causes many children's enthusiasm to die an early death. Why? Because shame is one of the first lessons that children learn in school. Oftentimes this happens when children want to be "different" from those around them. These children want to be recognized as individuals. However, public schools shun this.
Homeschool Children And Mass Society's Values by Brenda Hoffman
Schools may be able to prepare children to fit into the mass society or to help them find a set of values with which they could resist and reject the values of mass society. What this really means is that schools can teach children to believe what most people believe and to like what most people like. However, they cannot do both: prepare children to fit into mass society while helping them find a set of values with which they could resist and reject the values of mass society....
It's actually become so ingrained within society to think that in order for children to to live reasonably happy, useful, and successful lives they have to be considerably like most other people.
Socialization: Rules vs. Respect by Julie Ann Sih
Three weeks ago, I quoted Fred Frankel, Ph.D., on the rules of etiquette for joining other kids at play. Dr. Frankel contends that socially well-adjusted adults have mastered the intricacies of such social codes, from an early age. His book is chock-full of tables entitled "The Rules of Being a Good Sport," "The Rules of Being a Good Host," etc., etc....
So, if the type of social code-acquisition that takes place in unfriendly schoolyards is so darned important, why are homeschooled kids "happier, better adjusted and more sociable" than children attending institutional schools?
What About Socialization? by Rebecca Kochenderfer
This and other studies support the irony of the socialization issue in homeschooling that we have known for years, which is that traditional schools are actually more on a path of de-socialization. In traditional schools students learn to stay in a class to which they've been assigned and are grouped according to age and academic level, and generally with students from the same geographic area and socio-economic background....
The structure and reality of traditional schools are teaching students to be passive and compliant, which can follow the children throughout life. Children can learn to take abuse, to ignore miserable bosses or abusive spouses later on. In a traditional school someone else usurps authority.
TEN GOOD REASONS TO HOMESCHOOL by Greg Sherman, Ph.D.
Reason #6: Based on my observations, children in typical homogenous classrooms need to provide their own social modeling and behavioral feedback as they interact with each other throughout the school day. And in many cases, I have clearly seen that children are not usually the best teachers of constructive communication skills (listening, asking for clarification, evaluating without criticizing, etc.) or skills associated with conflict resolution. Stripping away the surface social behaviors observed among members of homogenous groups of children often reveals a more basic set of behaviors that are more in communion with the social order depicted in The Lord of the Flies.