For a commerce site such as Craigslist, the ratio of buyers to sellers is whats most important. The buyers wont come without items to be sold while the sellers wont come unless theres a market for their goods. If a marketplace was just starting out, it might want to link out to products in a similar marketplace to enhance its own offerings, in order to mimic the look of activity — the single measure which dictates if a social product works.

via Why the Web Hasnt Birthed a Prettier Craigslist.