Facebook turned on a key security feature by default on Wednesday that scrambles data sent by users to the company's servers, following similar moves in recent years by Web services such as Google and Twitter. Two years ago, the social networking site gave users the option of using TLS (Transport Security Layer) encryption, indicated by "https" in the URL bar. TLS is the successor to SSL (Secure Sockets Layer), a system that uses public key cryptography to ensure greater privacy between two parties.