What a beautiful dog! Thank you for taking her in until you can find her owners. I'd suggest you put signs on lampposts or such, indicating a small black dog has been found. If the owners are driving around looking for her, they'll see the signs.

Rather than posting your phone number in public, give your email address (or a throw-away one such as Hotmail). If anyone responds, require that they provide identifying information to prove the dog is theirs.

I have done this, and it works. Have reunited several lost dogs with their families. We live in the country and get many stray dogs. In cases when there has been no response, it's because the dogs were "dumped", but surely that little dear one is only lost.

Good luck! Also, watch the "lost dog" ads, of course, and place a "found dog" ad, too. There should be no charge for you to do so.