The socialization of dogs is a continual process, not a one time event. If you asked how I define socialization, it would be having the dog be socially stable in a variety of situations and around numerous environmental factors. One of the best services a human can do for its dog is to expose him or her a variety and multitude of situations, people, animals, scenarios, etc, and help teach them to be comfortable and confident with reinforcement of wanted behaviors.
Simply exposing your dog to another dog during puppy-hood can be helpful, but is only a small fraction of environmental socialization. Socialization is broader– it includes being able to walk down a street and politely see another dog, being able to greet another dog, being able to sit calmly in a public setting, etc.
Utilizing services such as daycare can be a crucial first step in teaching the dog how to be socially stable within a group of dogs. Daycare is a safer alternative to dog parks because all the dogs in daycare have been thru a behavioral evaluation and have shown vaccination records. Remember, having a bad interaction with another dog can be just as bad or worse than no socialization.