Naturally this depends on what is meant by "socialized". I am not sure how this could be researched, however. Number of friends? Hostile or amicable relationship with parents? Hostile or amicable relationships with peers? Frequency of contact with members of the community? Volunteerism rate? Suicide rate of homeschooled vs. traditionally schooled? Violence and bullying in traditional schools vs. homeschool groups?
However, here are some differences I've noticed, as a homeschooling mother:
Age Cohort & Peer vs. Family Orientation
Most schooled children spend most of their time with children their own age, usually within a year of their age, with a few adults teaching or supervising. Homeschooled children tend to spend more time with their families, siblings and children of different ages. Typical homeschool groups include kids of a variety of ages, from newborns through teenagers, and often you see four-year-olds working side-by-side with 10-year-olds. Related to the age cohort difference is the result of that stratification by age. The tendency for schooled children is to be primarily peer-oriented vs. parent or family-oriented (see below). There was a study done which indicated that mixed-age groups of children were significantly less likely to exhibit bullying behavior. Children interacting with other children at least 3 years younger saw themselves as protectors and role models, rather than as competitors or rivals.
Personally I believe our society is broken in that people mainly associate with people their own age. My relatives in the Philippines, if they threw a party, would include everyone -- babies, kids, teenagers, people in their 20s, 30s, 40s -- and grandmas in their 80s. This was not unusual, and I think, the mark of a healthy society. However I rarely see this kind of intergenerational mixing in the States, except with first generation immigrants.
One-on-one attention
Parents also know their children, and can adapt the pace of learning, the subjects taught to the individual child. Traditionally schooled kids have to keep to the 2nd grade, 3rd grade, etc curriculum as taught, whereas homeschooled kids can go faster or slower as needed. As has been noted in the recent article regarding Khan Academy in Wired (http://www.wired.com/mag
Community participation
We are able to spend time every week at a nursing home, in conversation with the elderly residents (we visit one resident in particular that we have a close relationship with, but have adopted the whole place). Children and the elderly are almost never seen in our daily lives and are missing from civil society. As such their needs are not taken into consideration in many decisions we make regarding public life, nor are children given real responsibility in this regard. I also think to be well socialized one should contribute to society by doing volunteer work -- and not putting stamps on envelopes in uptown offices or throwing gala fundraisers -- but by serving in soup kitchens, or cleaning out the bedpans of the very old and sick. John Taylor Gatto wrote an essay about kids doing volunteer work, and how it fundamentally changes ones relationship to the community, and one's self within that community.
Closeness to Family
After "better education" the reason most frequently cited for why parents homeschool their children is in order to have a closer family. Peers, media, and other influences commonly drive a wedge between children and parents and homeschooled children tend to have a closer relationship with parents and siblings.
Determining who in our society is 'well socialized' is subjective. But a friend of mine in the tech industry asked me "Why is it that homeschoolers are so much better socialized than other people?" He mentioned a woman at his company who was always sent out to talk to new employees, meet new customers, talk to "problem" clients. "She can talk to anybody," he said. It might just have been the woman's personality, but one of the reasons I decided to homeschool was I met a friend's 12-year-old daughter, who spoke to me without fear, as to another person, and not as an adult as most kids do. She was talking about the radio show that she DJ'd on a local station and knew so much about music, which she clearly loved. She was empowered to pursue this interest by her parents, as a homeschooler.
There is a book called The Well-Adjusted Child, about homeschooling and socialization which I reviewed on GoodReads with some notes from the book.http://www.goodreads.com/