This is a question that homeschoolers hear quite often, so I'm glad you asked. When my 10 yr old heard someone ask how much socialization she got, without missing a beat she answered, "A lot!" One homeschooling mother told me that when she started homeschooling, she signed up for every class and activity she heard about. After some period of time, she and her children were exhausted from running around all the time and realized they could stay busily engaged in social activities every minute of every day if they wished. They became much more choosy and found a balance between time spent on learning at home, learning with others, and time spent just having fun. My mother, a former school teacher, was initially quite concerned about the socialization aspect of my children's experience, but I think she has been pleased with how things are turning out.
Here are some useful statistics from the National Home Education Research Institute:
Dr. Larry Shyers observed children in free play and group interaction activities. Conventionally schooled children had significantly more problem behaviors than did the home educated. This is probably because the primary models of behavior for the home educated are their parents.
Home educated children are more mature and better socialized than are those sent to school, according to Thomas Smedley's personal interaction and communications approach to understanding socialization.
Dr. Gary Knowles, of the University of Michigan, explored adults who were home educated. None were unemployed and none were on welfare, 94% said home education prepared them to be independent persons, 79% said it helped them interact with individuals from different levels of society, and they strongly supported the home education method.
Additionally, Dr Knowles published a paper called, "We're Grown Up and We're OK." He brings up the very interesting idea of the difference between socialization and social development. He defines socialization, being what our home schooled children are supposedly missing, as "the process of fitting young members of society into the mould cast for them by the adults of society." Social development, rather, is, "defined as having psychological and emotional dimensions," and can be nurtured beautifully within a homeschooling context. Explained this way, our choice to homeschool may make more sense to those who would doubt our ability to provide our children with an appropriate social experience.