I think we do see a lot of this today: Uber and Lyft already have incorporated ride sharing and I use that option every time -- when I'm in a city I frequently have another rider or two. I'm curious to see how this will increase as ride sharing density grows.

I think people avoid public transit because it is necessarily slow due to the multiple stops. They may also avoid it because it is too accessible. Sharing a vehicle with other commuting professionals is a vastly different experience than what you'll find on most public transit.