The problem is as follows:
On day 1, a person will call two persons; these will, in turn, call other people on day 2; generally, if a person gets called on day n, they will call another set of people on day n + 1.
There are two kinds of people: sociable and unsociable. Sociable people always call 4 people; unsociables always call 2. Also consider that every person calls exactly one sociable person.
The problem then is to find how many people will call on the nth day. I have a solution for this but I think it is kind of artificial. I would like to see more solutions to this.