The black points plotted below represent the number of active Facebook users [i]u[/i], in millions, [i]t[/i] months after December 2004. For example, in December 2005, there were about 5.5 million active users, so the point (12, 5.5) is shown. The data ends in April 2012. Also shown, in the form of three graphs, is the solution of an [color=#008800]S[/color][color=#0000ff]I[/color][color=#b20ea8]R[/color] infectious disease model. (See [url=http://en.wikipedia.org/wiki/Compartmental_models_in_epidemiology#The_SIR_model_without_vital_dynamics]Wikipedia[/url] for details.)
You can drag the two [color=#ff0000]red sliders[/color] at the bottom left to change the two parameters of the SIR model. You can also drag the [color=#ff0000]red point[/color] on the "[math]y[/math]" axis to change the initial number of susceptible people (in millions).