Off the top of my head: The pessimist in me says it's so each of them can create a more complete user profile for marketing, advertising, and tracking. My mor

Off the top of my head: The pessimist in me says it's so each of them can create a more complete user profile for marketing, advertising, and tracking. My more optimist side says that Facebook, Google, MS, and Twitter are all tech giants filled people intimately familiar with the web and its immediate future, and the majority of them are genuinely trying to improve the internet. As usual, the answer probably includes both factors, and likely others.