I'm not asking from a business sense exactly, but for example, both reddit and Twitter are completely open source. It is my understanding that at least the large majority of their profit comes from advertising on their website. So what exactly is to prevent someone from copying their code and making their own website with some small but effective changes?
I ask this because I have a website, and I want to make money on it through advertising. I also want the code to be open source, just to be nice, but I don't want this to happen to me. My site is not (and will probably never ben close to) an institution like Twitter or reddit. I'm not sure if they're just so big they're not worried about someone copying their site, or if they actually have some protection in spite of being open source.
reddit uses CPAL, and Twitter uses the Apache license -- does this offer the protection they need or is there something else I'm not getting?