First of all, "socialization" is very different from "socializing". It sounds like you're talking about "socializing" - which means hanging out with others. Socialization is the process of acquiring social skills that are necessary to interact with others in our society.
Secondly, kids don't necessarily need as much socializing as we think they might, even if they appear to really crave it. Just as a comparison, kids will crave and beg for chocolates and potato chips and pop and most would gladly eat it all day every day if given the chance. That doesn't mean it's the best choice for them. It's a hard concept to grasp, since our society is SO entrenched in the "playing with large groups of same-aged peers is absolutely necessary" mindset, it's hard to even consider the possibility that maybe it's not only unnecessary, but perhaps even potentially harmful.
I'm not saying it definitely IS harmful. I'm just saying that, just because a child wants something doesn't necessarily mean it's good for them; so we should evaluate whether it actually is okay before giving it to them; and that should apply to socializing as much as to anything else.
To understand the potential negatives of too much peer socialization, I really, really recommend "Hold on to Your Kids: Why Parents Need to Matter More Than Peers" by Neufeld and Mate.
Again, this isn't to say that you should AVOID socializing, far from it! But it's really not something where we have to WORRY about our kids getting ENOUGH socializing. It's like candy, sort of... it's fun, it's enjoyable, but you don't need to overdo it.
Through most of human history, kids spend most of their time interacting with a wide community of people of all different ages, there weren't large groups of same-aged peers to spend so much time with and it wasn't seen as a really, really important thing.
Most homeschoolers find that the socializing that happens naturally, with neighbours and family and through various activities, is plenty. I wouldn't join an activity JUST for the socializing, but it will be a side benefit of activities you will join anyway. Getting together with other homeschoolers whenever possible is also great. Not so much because it "meets a need for socializing", because as you can probably tell I don't think that need is very strong or very real.
It's more because it gives kids a kind of home base, we all want to feel we belong in a group (it seems hard-wired into humans) and not just be outsiders. If you're the only homeschooler you know, you feel like an outsider. A group of homeschoolers gives you a place where you feel at home -- even if you don't actually get together all that often.