Just gonna comment on my 2 current dogs because they are the 1st puppies I have raised in years.
If we are talking in terms of early socialization as just "exposure to" (sights sounds etc...), then I consider it successful. Nothing really phases them but I am still careful with them. We have a community of minunites (sp?) who use their horse and cart to come to town when they need to shop. The first time the boys saw one, and it was physically close, I'd say about 10ft from us, they were curious but nonreactive. I think this is because I have always taken them with me places and they have seen so many things.
If we are talking about socializing with other dogs, I haven't done that. Everyone I know with dogs, doesn't have a dog I would want my dog to grow up to be like