The application of disease-like dynamics to [Online Social Network] adoption follows intuitively, since users typically join OSNs because their friends have already joined. The precedent for applying epidemiological models to non-disease applications has previously been set by research focused on modeling the spread of less-tangible applications such as ideas. Ideas, like diseases, have been shown to spread infectiously between people before eventually dying out, and have been successfully described with epidemiological models. Again, this follows intuitively, as ideas are spread through communicative contact between different people who share ideas with each other. Idea manifesters ultimately lose interest with the idea and no longer manifest the idea, which can be thought of as the gain of "immunity" to the idea.
The researchers used Google Trends search data to measure the popularity of Facebook over time. Again, this idea has precedence; the popularity of Google searches correlates with the spread of other trends.