But even having found a new companion for my pig, I still look at Craig's List, several times a day, looking for "free guinea pigs", with the specific purpose of explaining the dangers of "free".
Having found this thread a few days ago, I also include a link to it in my email.
I don't know if it always works but there have been occasions when I've emailed someone and then saw that a rehoming fee had been added to the Craig's List page.
So, although I don't know how often it works, and surely sometimes it doesn't, sometimes it does.
A good hint I saw on CL was to trust your instincts when placing a pig. Unless you know or know of the person be wary of the person who offers to pick up the pet--you may want to bring the pet over yourself so you can see where the pet is going. Also, vet's offices might be a good place to advertise--that way you and your vet might get a better idea whom the pets are going to. Just my two cents.
The other two staffers who did the adoption asked me a lot of questions about diet, proper handling and guinea pig behavior and I answered their questions the best I could. The result was them asking if I wanted a job. Of course I declined, largely because guinea pigs are not the only small animals they have and I know very little about rabbits, chinchillas or most other small ones. I could volunteer, and I still might if they get anymore guinea pigs.
The reason I adopted Sam and Dean is because they were on their second shelter stay. The Nevada Humane Society only keeps them for 60 days and then transfers them out to another branch. Sammy and Dean had been in shelter status for 81 days. I know that they have a no kill policy for dogs and cats but I don't know if that applies to guinea pigs. I didn't want to leave it to chance, so I took them in. Turned out to be good for them and me. They have a loving home and I got 2 of the cutest little piggies.
If I had the means, I would open up a rescue of my own for guinea pigs only. I'd take every one of the "free to a good home" piggies as well as the ones that come to the Humane Society and make sure they get the best home and are properly taken care of while they are at my shelter.
Realistically, however, You could offer to help that humane society anytime they have guinea pigs come in. Take in decent pellets and hay, along with a couple of Lynx's care booklets and offer to treat the pigs for mites or sex them for the staff. Also write down the address for the cage site so they can pass on that information to prospective adopters.