That's what I get asked a lot when I mention that I plan on homeschooling M. I dunno, honestly, but so far we aren't suffering from lack of it. It's not like we stay home all day every day. And everywhere we go, there are people of all ages and we get to interact with them in various settings and in many different situations. Which, I think, is a more wholesome and balanced socialization that being in the same room with 20 other kids the same age day in day out.
Several times each week we have playdates with M's friends. Sure, for now it's more of a one-on-one playdates, but hey, he's only 3 years old. 3-year old don't play as a group. Instead, they end up either playing alone or with just one other playmate.
We do have a huge sharing problem though. I mean, as long as we're out and about and the toys aren't M's, he will take turns and share. But inviting anyone over to our house is simply not possible. I mean, we do invite kids over and sometimes things go just fine, especially if it's an older child (they seem to be more patient and understanding) or girls (since they aren't too eager to play with blocks, trains and many other of his toys).