Couchsurfing has a convention that if you're going to surf you should really host as well, unless there's a good reason why not. It's not enforced in any way, but there's definitely an expectation from many hosts that you will at least fill out a half-decent profile and try to build up some decent recommendations from people in your home town by attending a few social events or whatever. In other words, if you haven't hosted a few people, or otherwise gotten at least a couple of recommendation (and taken some time to flesh out your profile to make yourself seem at least a little interesting), a lot of hosts won't take you.

CS is fantastic, but it's not simply about free accommodation: to get the most out of it you need to put in quite a bit of effort too (which is as it should be).

Airbnb seems to be for people who don't really want to go that route.