From a Wall Street Journal interview with IAC head and former media mogul Barry Diller:

WSJ: Companies have struggled to make money in online video and social networking. What is the outlook for advertising in these areas?

Mr. Diller: You really want to get a headache? Try to understand Internet advertising. Social networking advertising is being discounted because there is so much inventory [of available ad spots], and because methods have not yet been found to make it very effective. Will that get figured out? I absolutely believe it will. What form will it take? Absolutely unknown.