The product for YouTube and Airbnb is the community, not the actual code. Although for YouTube they might want to keep some of the video processing stuff to themselves. As for Airbnb a clone wouldn't take it down because the community is the product. Same can be said of Craigslist.
No one will see the value in doing what you're doing and want to clone you until then. Even if someone saw the genius in your approach early on and decided to clone you and start competing with you, you have to be confident that your version of your idea will continue to be better than their version. You still have all the advantages of branding, community, and users, whereas their version has no advantages except perhaps a lower price (which doesn't apply to many products, like your AirBnb and Youtube examples).
