My puppy, Cassie, is 5 months now. She has made a lot of progress with socialization - less barking and fearful responses. In our case I can say it's been a matter of still having some reactions - good progress, with an occasional step backwards.
My vet wanted me to wait an extra 2 weeks, before taking my dog out in public. My goodness, she did a lot of barking: traffic, her reflection in a window, people in general and other dogs caused her to bark. At 5 weeks, she is doing a lot better, but she is still a work in progress
last week, in obedience class, she started squealing, when a large Newfoundland dog took a step towards her. The other dog wasn't aggressive, just friendly. The dog was still a couple feet away, when Cassie reacted. I think the large size of the other dog scared her.
A few days ago, we took a walk in the park. A small friendly dog (1/3 her size), tried to approach her and give her a sniff. Again, Cassie, started squealing. She has previously gotten close and sniffed and had friendly meetings with dogs in class, as well as the park.
In our example, I'm thinking that Cassie's socialization isn't just following a straight line. She has good days, and then some occasions of showing some fear. A few days ago, she barked because she noticed someone walking behind us, during a neighborhood walk. That was something I'd like her to bark at. Where I live, it's good to know that someone is behind you and to be cautious. Sometimes, I've proactively placed Cassie in a "sit - stay" position to get her out of the way and let another person or person with a dog pass. Generally, when I've done this proactively, she doesn't bark and then I give her a treat when she is quiet.