A classic, with illustrations.
A much better analogy for digital piracy is sneaking into a theater to watch a movie. You are not stealing a copy of that movie, and the theater is free to show the movie to others. But you are stealing revenue that the theater would have earned had you rightfully purchased a ticket.
So when you pirate music, video or software, you are stealing income from the seller. You are receiving something of value without paying for it.
READ THE FULL POST AT DEVTOPICS:
http://www.devtopics.com/no-silly-piracy-is-theft/
The Trichordist 
Not quite. You are entering the movie theatre and making a copy that you then distribute. Thus you not only take the revenue once, you take the revenue from the movie maker, the theatre owner, the theatre employees and so on. Viral systems spread disease virally. That is what network effect is.