With an asking price of $350,000, the small home at 16 De Long St. is one of the least expensive homes on the market in San Francisco. And, to the naked eye, it looks like a falling down shack. But in this city, not all shacks are created equal. Here, there were more than 5,000 earthquake shacks built after the 1906 trembler that leveled a good part of the city and left more than half of its population homeless. A few of the shacks remain, and they have a loyal following of devoted history lovers.