There were a couple of German car-via-internet-sharing startups (tamyca was one of them, seems they're still in business) and I tried them once. My impression was at the end that I payed not much less than when I would've rented from a commercial car rental company, but with much more overhead.

So that was the experience for me as a customer. With airbnb it's completely different. I feel I often get more from the things I want (e.g. kitchen access is very common in airbnb's, no Airbnb ever tried to charge me for wifi) and the prices are often massively cheaper than hotels.