Paul, you're completely ignoring my main point by focusing on this idea that I'm suggesting ad-based models don't work. I'm not saying that.
What I'm saying is that because consumers that don't invest out of pocket are fickle and that ad-revenue follows consumers, valuations of companies that survive solely on ad revenue should reflect this.
Right now they do not, hence they are overvalued. When a company is overvalued, it's not a good time to invest. The fact that there continues to be a market is based on the fact that Facebook, or a site of equal stature hasn't actually had to justify its value.
Once it does, the house of cards will adjust a little.