Viral effects are advertising with low/no staying power because people lose interest and move on to another bright shiny new thing. Network effects have staying power because people in a network tend to all have similar interests.
"The Facebook" had an initial viral value that bootstrapped its network in a way that, say, HotOrNot did not.
Short Scenario: Living with an Girlfriend, a normal situation, _you_ are completing her spoken sentences before she could finish (not a 'what was that word'-situation) -because you can and may do what everyday-internet-practice does since...^^