I totally agree that you cannot introduce your dog to too many people or dogs BUT I do not believe in off-lead socialization, for many reasons.
1. Puppies can communicate quite fine with a lead on
2. If the all pups have a lead you can break up any fight / bad manners very quickly
3. You still have good control of your pup and can still teach them right from wrong manners around other dogs
4. I have NEVER met a trainer, vet or behaviorist that I would rely on to be able to keep an eye on and read 3 or more puppies at once (who are all running around like idiots)
That being said of course it is important for dogs to learn appropriate play as well so I allow my dogs to play with dogs that are friends of mine and that I will see often so they form a friendship and learn to play well with each other.
Also on that I don't like the idea of my dog being overly friendly with every dog they see on the street as NOT every dog is friendly in return. I prefer my dogs to ignore any dogs they don't know so if they ever do get out without my supervision HOPEFULLY they will stay within their pack and ignore any other dogs they might come across.
"In order to really enjoy a dog, one doesn't merely try to train him to be semihuman. The point of it is to open oneself to the possibility of becoming partly a dog." - Edward Hoagland