An anonymous reader writes: The Internet's biggest players — including Facebook, Google, Yahoo and Bing — are turning on IPv6 today as part of the World IPv6 Launch Day challenge coordinated by the Internet Society. But which websites are not ready to support the next-gen Internet Protocol? Well, Twitter, for one. Other high-profile sites that aren't among the 3,000 participants in World IPv6 Launch Day are business-oriented social networking site LinkedIn, e-commerce giants Amazon and eBay, online reference Wikipedia, and classified advertising service Craigslist. Neither Tumblr nor Pinterest — two popular image-sharing websites — is participating in World IPv6 Launch Day. Nor is PayPal.