Those are the dogs that I adopt, too! The dog I have now I adopted when he was about 2 1/2. It is now 1 1/2 years later, and we are still working with his fear issues. However, they have gotten better. I work really hard to socialize him (he goes to doggy daycare, we go to the dog park where he can meet other dogs and other people, etc.), but there are times and days and people of whom he is still just terrified. And I just have to accept that, support him, and move on.
One of the things that I have found is that people assume that because he is cute (actually, so ugly that he's cute!), they should just be able to walk up to him and pet him. I have to tell people that we see how they should approach him. I anticipate that this will be a long process - at least another couple years, but that we will get there.
My last dog was so badly abused that the vet at the shelter suggested they just put him down - but we worked together and lived happily together for 15 years after I adopted him. He never was overly friendly with strangers, but he was a wonderful, loving dog with me. I think in his heart, he knew I had saved him, and he went above and beyond and saved me, in a lot of ways, too.
As someone else suggested, it's important that there is a safe place for your dog to go when he is scared. I highly recommend a kennel. Put him in his kennel before people come over, and after a while go over and open the door. But don't pull him out, let him come out when he's ready. And if he's not, that's ok, too. It's ok for a dog to choose to stay in his kennel until he feels safe. I also second the recommendation of calling a behaviorist. I've worked with one for this dog, since he had some fear aggression issues, and it has been very illuminating.
Good luck and enjoy your puppy.