We don't have a homeschool co-op here. But my kids are very involved with other children through homeschool activities such as field trips , board game days, monthly open swim, etc. Our group focuses more on social activities than academic, though we often do short term academic programs with other families such as a science class or book club. I really have to work to have us home enough to actually get some schoolwork done! There are so many opportunities to do things with other families that I am now trying to cut back.
We also have no shortage of playdates and informal hanging out. My 8 yo plays webkins online with her friends when they can't get together, my 12 yo spent over an hour on the phone with a buddy last night. They're really not so different from schooled kids in the way they interact with others.
I highly recommend this book about homeschooling and socialization:
http://www.bellaonline.com/articles/art27055.aspHomeschooling isn't for everyone, but I hate to hear that social concerns are keeping someone from trying it. You do have to be proactive about getting together with others, but there are homeschoolers all over the place these days.
good luck!
