Some certificates come blank where the user's name would go.
Each tweet's rating, from one to seven, is represented by little blue starbursts next to the user's name, almost like a Yelp rating.
On the Romney site, Mr. Mayer said, he found that a number of pages included the user's name in the page title.
Facebook says it turns over content created by users — pictures, posts, messages — only after it is served with a search warrant, though a subpoena suffices for basic subscriber information, including a user's name and e-mail address.
Using that information, the F.T.C. said, third parties could obtain the user's name and other personal information and use a file placed on the user's computer to view a history of Web sites the user had visited.
It could allow a criminal to redirect Web traffic secretly, so that a person typing a bank's actual Web address would be sent to an impostor site set up to steal the user's name and password.
Once a site collects a user's name and address, it can buy other information about him or her from mailing-list companies or, if it already has a database, it can examine its own customer files.
Having a user ID allows someone to look up that user's name and any data posted on that person's public profile, like a college or favorite movies, but not information that the user had set to be visible only to friends.
Google said it had been fighting the broad use of subpoenas by requiring search warrants for detailed personal data — beyond things like a user's name, location, phone number and time that an e-mail was sent — even if the law seems to say a subpoena is enough.
While the data collected by the cookies would not contain the user's name or personal details, privacy campaigners have long pointed out that the pattern of a user's web browsing allows a picture of them to be built up which can led to direct identification or profiling so precise that it leave little doubt about their identity.
We replaced every user's name with a different letter from the alphabet for reasons of anonymity.
