The socialization question is an extremely common one, and it usually indicates that the person asking it thinks unschooling is somehow "school at home" or something similar. This is frustrating, since it's exactly the opposite of the truth: People unschool their children so they can be exposed to a wider range of social experiences than those provided in the narrow and artificial environment of most schools.
I think pg's essay on why nerds are unpopular has some good insight into much of the "socialization" that goes on in school: http://www.paulgraham.com/nerds.html
Dylan Klebold and Eric Harris would not have been serial killers if they had not be so mercilessly bullied in school.
Causation for that has not been rigorously demonstrated. But, agreed, the school social environment they were in is described by press reports as having been toxic even before they brought their weapons to the school.
I think pg's essay on why nerds are unpopular has some good insight into much of the "socialization" that goes on in school: http://www.paulgraham.com/nerds.html