That's not the point, though. The point is that due to network effects (which lock users in to a platform), ads are monopoly-priced. But network effects are no

That's not the point, though. The point is that due to network effects (which lock users in to a platform), ads are monopoly-priced. But network effects are not super well understood and platform collapses do happen. So we know the ads market to be large -- or at least people on HN are testifying to that effect, but we don't know that Facebook or MySpace or Lycos get to appropriate it to a long enough period of time that current valuations make sense on a no-bullshit NPV basis. A fair point was made elsewhere that multiples (which are more or less like stock market folklore, but still allow for comparison across multiple industries) in these companies are relatively modest.