Make sure you don't put any identifying info in your ads. I would just say "found puppy" with a general location and make the owners tell you what they are looking for.
I go/recommend these routes:
I make a flyer for AC, for all the local shelters, fire departments (lots of people bring them strays, oddly enough), police departments, vet offices, and I post flyers in the area I found the animal. I also put ad in the paper (many papers will do lost/found for free) and on Craigslist (but I use an extraneous email so I don't get flooded with spam).
I put out ads for a pit bull I found in 2004 and I am not exaggerating when I say I must have gotten 50 phone calls. Some people really were missing their dogs, but I am sure a great many just wanted a free pit bull. Some of the calls were down right funny. I never found the dog's owner but I did get the little boy cleaned up, fed up, and vetted then got him a great home where he is still happily spoiled to this day.
If you don't find his family, I wish you luck finding a foster situation.