Best way to find missing dog?

It's been almost 4 days and I've posted to facebook but I cannot seem to find anything on where he'd be from anyone, people have said they've seen him, but (a lil rant here) why wouldn't you keep him? I genuinely had someone take a picture of him in her home with her other dog, he's really friendly too. Thing is, my Mom is going crazy because he's gone and I really don't know what to do, I'm trying to hold onto hope here but it's hard. He's chipped too, but my worry is that someone just took him and he's just gone for good. Any ideas?, really anything would help.