I’ve been a member of Postcrossing now for six years. I enjoy sending and receiving postcards from around the world. I’ve sent almost 4000 cards and I’m in the top 100 members from the USA. One thing I’ve noticed is that some countries are per capita much more likely to do Postcrossing than others. Germany has the most users followed by Russia then the United States.
I think there is an opportunity for someone to learn a lot about the viralness* of activities by studying Postcrossing and trying to discover the variables that effect it’s spread through various countries. For instance one would think that English speaking countries would be heavily favored. but in fact France has sent more cards that the UK.
Maybe someone from the CDC, the NIH or a university student studying epidemiology.
* If this is not a word it should be.