What other people are doing, who are doing it. Synchronously, asynchronously, the resources one are using. For example, well-defined borders, not very available on the web. Co-occurence in longer sequences. Proxies and caches can be used to investigate individual behaviour.
Traffic analysis can give data of value.
Privacy, security, presence, community His lecture started a long and interesting discussion among participants about issues of privacy, secrecy, presence, community. In a society which perfect privacy, no one would know anything about anyone else, all messages would be written by pseudonyms. But is this the kind of society people want. Is it not rather true that people like to see each other, know about each other, understand about each other. This speech presented the Web4Groups system and how ratings could be added to it.