I looked it up before posting. It's illegal in 48 states, including California where most of these companies are headquartered, and every state where major cloud data centers are located. This makes it effectively illegal by state laws, which is the worst kind of illegal in the United States when operating a service at a national level because every state will have slightly different laws. No company is going to establish a system that allows users in the two remaining states to exchange revenge porn with each other except maybe a website established solely for that purpose. Certainly Snapchat would not.

I've noticed recently there are many reactionary laws to make illegal specific things that are already illegal or should already be illegal because of a more general law. We'd be much better off with a federal standardization of revenge porn laws than a federal law that specifically outlaws essentially the same thing but only when a specific technology is involved.