Maximal number of connections of N nodes is equal to N(N-1)/2. Intuitively it can be understood as follows. Each of N nodes can make connection with N-1 other nodes, as connection to itself would be trivial. Each connection has two nodes so it should be counted only once, thus division by 2.
This is not equivalent to everyone having a FB account. It is equivalent to everyone having everyone else as a FB friend.
