"Oh, socialization is not a problem. I take him into the bathroom once a week, beat him up, and take his lunch money."
After they recover from that smack in the face, I get serious, and explain that, no, I don't want to raise a weird, hermit-like kid. That's why we're involved in Cub Scouts, he's in all sorts of sports, he has many friends (homeschooled, privately educated, and public schools) with whom he plays regularly. The difference is this: In my son's case, we know where he's at, whom he's with, and have a high degree of confidence that when he's not physically with us, he's under the supervision of another adult we respect. 24 hours a day.
In the past 2 years, he's been on trips with me and/or my wife to Florida, Alaska, Minnesota, and California. We've taken shorter trips to Washington, DC, Gettysburg, Philadelphia, Mount Vernon, on and on. There's a chance he'll get to go with his grandparents to Ireland and England next year. We're planning a camping vacation with my wife's sister's family (yep, they're homeschoolers, too) to the Dakotas and Yellowstone. He either does schoolwork on the trip, or he works hard to get ahead and enjoy a break from normal classwork. On all of these trips, there are educational activites and just plain all-out fun.
I'll bet those extra-curricular activities stack up pretty well with whatever the local second grade class is doing.