I have always given it back.
I suppose most people would act like that as they have the person in front to whom they would create a problem.

If you think of finding 50$ on the street I suppose more people would keep it (no Pet Talkers of course ).

One day I found a wallet on the street- with money and addresses and all- again I gave it back. The person had put it on top of her car (yes that really happens ) and had lost it that way.