Network Effect (definition shamelessly copied from Wikipedia): A network effect is the effect that one user of a good or service has on the value of that product to other people. When network effect is present, the value of a product or service is dependent on the number of others using it. The classic example is the telephone. The more people who own telephones, the more valuable the telephone is to each owner.
A year or so ago, there was some hubbub when Facebook bought Instagram and started to change its privacy settings. Instagram had been a small private company that did nothing except let you take pictures and post them. The app was (and still is) well done, easy to use, and easy to love. A large number of people I knew were on Instagram, and I enjoyed Instagram “likes” (I forget what they were called) and comments. It had been a decent, innocent company, until Facebook came along.