Recommendation engines…more people shop at Amazon, the better their recommendation engine gets…which, in turn, helps people buy more stuff at Amazon.
eBay would not be a particularly useful site if auctions were not competitive. Each individual makes the auctions grow more competitive, pushing up the prices of bids on items….thus more people come
On Wikipedia as the number of editors grows, the quality of information on the website improves, encouraging more users to turn to it as a source of information. Some of the new users in turn become editors, continuing the process.
Google Adsense: The more blogs Google AdSense can reach, the more advertisers it will attract, making it the most attractive option for more blogs, and so on, making the network more valuable for all participants.