I was in the store yesterday, and overheard a couple of backpackers asking about public transportation to Yosemite. I was able to connect them with one of the people in my group was headed back to the Bay Area in the morning, and had room in his car.
A couple of years ago, I was able to give a couple of JMT hikers a ride back to SF. In that case, someone else in our hiking group had met the backpackers coming down the trail to the Portal and played match-maker.
I am sure that there are many in similar circumstances, but never connect. Almost everyone going back to SF will go via Tioga Pass. Some will have room for an extra passenger or two and will be willing to provide a ride. I ocassionally see a posting for rides wanted, but it seems to me that a separate listing for rides needed/available may fill a need.
Ralph

ps. Our trip was a success. All twelve of us made it to the summit, and got back down before the kitchen stopped taking orders.