This is a topic that interests me greatly!
Socks I approach like any other technical gear in the woods. When in the rough, I prefer a high-tech, breathable, moisture wicking base layer, like coolmax or some other thin sock, over that I like wool, if needed, for warmth. I suppose any high-loft, crush resistant material would do, but the fact that wool continues to insulate well, even when wet makes it a superior choice for the feet (which are prone to wetting more than other parts of the body). For a waterproof layer, I prefer it be built into my boots (gore-tex) or by using rubber galoshes/overboots.
The drawback to wool is that it tend to hold water and takes a long time to dry out. I think, for the homeless that may not understand the important connection between moisture and hygiene, it may not be the best choice in material. I think dryness would be more of a concern for the homeless than warmth. Afterall, there are a ton of insulating materials that can be found to wrap around the feet in a jiffy to protect the feet during a long, cold night if you are on the street, but it might be harder to keep the feet dry and free of fungus/infections. Just my thoughts, as I am no expert on the logistics of being homeless.