I won't argue about the efficacy of home-schooling for academics, nor will I discount the value of family time. As such, the socialization thing is way overblown by the NEA: who, it must be remembered, have ushered in a system whereby "socialization" apparently means giving in to the worst manifestations of modern trash culture.
Still, I've seen many homeschooled kids, especially boys, who really do miss out on certain important social lessons. My experience has been that many homeschooling parents are very protective of their kids, and the primary teachers are their (protective) mothers, who very tightly restrict the people with whom their kids interact.
For boys, this often leads to situations where they miss out on learning "guy stuff" -- those stupid-seeming games where the main point seems to be inflicting pain, and laughing when you get hurt, and doing it with a group of like-minded guys.
Guys will understand the fun of a game like, say, "butt ball," the point of which is for one team to taunt another by waving their (clothed -- you'll soon see why) rear ends at the other team, who attempts to hit same with a whiffle ball thrown as hard as possible from close range.
Churchill once said, "nothing in life is so exhilarating as to be shot at without result." OTOH, there are very important lessons to be learned from getting stung and laughing about it with your team mates. And it's remarkably difficult to imagine a group of home-schooling mothers allowing such a game to take place.
If nothing else, a more public education (i.e., not necessarily public schools, but with relatively uncontrolled interactions between kids) provides access to to guy stuff, not to mention an education about how to deal with people who are not just like you.
There are a host of clubs and group activities that parents can involve their kids with such as sports, Boy Scouts, computer clubs, self-defense, summer camp, etc., where kids can still learn social skills by being around others. And they're far safer than government schools.
I can relate to your comments on over protective mothers. I think a problem that arises and should be dealt with is the amount of time the boys spend with mom and sister/s. There needs to be concerted effort to allow boys to be around other boys and men. My husband started up a Tuesday night group for homeschool boys and dads to spend time together--they learned algebra together and did some other stuff. My son and husband went to Boy Scout camp for a week this summer and I was so thankful that my son was getting a whole entire week with boys only--then I found out they allow women leaders! (better than gay anyway, right?) But, you know what I found out is that the Lord made my son to grow into a man and he is becoming just that despite all the time he spends with me and his sister.
I have "fond" memories of fifth-grade recess "Kill the Kid with the Ball" games. How do you play? Whoever has the ball gets pig-piled on and pummeled. It was OK if you kind of moved along with the herd and pretended to try to go for the ball every once in a while.
Unfortunately, sometimes kids who had no interest in playing were "given" the ball.