Honestly, I point out all the things that my DD is involved in with other kids (Sunday School, gymnastics class, Homeschool group) on a weekly basis and explain that part of the reason we pulled her out (not the total reason, but it's one of many reasons) was because the school was not providing a social outlet for my child anyway. They came to ME asking ME what I was doing for social outlets outside of school and specifically said that she needed outside social outlets because the school wasn't providing much social time. I was also concerned that the only social time my DD had was during lunch time, which came at the expense of her NOT EATING OR DRINKING anything all day long. They didn't go out on the playground or have any unstructured free time during the day. So, what difference does it make if my child is educated at home or not? She's not losing out on those social interactions.
Besides, why does a child need to be interacting socially ONLY with his/her age-mates? My DD does a fantastic job interacting with the kids of various ages at our homeschool group. In fact, she has been interacting quite well with a 10 year old and my DD is 6. And why is it inappropriate to learn some of her social skills from other adults as well as her age-mates or kids of varying ages?
You could very well ask what the questioner is defining "socialization" as being. And if "socialization" simply means interacting with people other than your immediate family, list off the various activities that your child will be involved in with other kids/people. If "socialization" means interacting with kids of the same age-group, ask why kids need to interact only with their age-group, when we as adults can interact with people who are years older or years younger than we are and that's perfectly ok. We as adults don't go up to someone, ask how old they are, and then refuse to talk to or become friends with that person simply because they aren't the same age. That would be crazy. Why can't our kids be involved with other kids?
One thing that might be really beneficial to you is to join up with a homeschool group in your area. I know that my DD likes going to our homeschool group because then she's interacting with other kids who are being educated in the same way she is. It's a great way for her to realize that she's not the only homeschooled kid around and that it's not "weird" or "abnormal".
Stay in contact with what school friends your child has made and try to set up playdates when you can. I think it's important for kids to keep those positive connections and don't feel like you need to make excuses for your decisions. The public school isn't working out for you or your child. I try not to slam the public school system to other people who are using it, simply because they may have no other choice...or maybe it's working for them. But, I just say that it wasn't working for us and we decided to go another route.
I hope homeschooling is a very positive experience for you all!