Ten billion Twitter messages take up little storage space: about five terabytes of data. (A two-terabyte hard drive can be found for less than $150.) And Twitter says the archive will be a bit smaller when it is sent to the library. Before transferring it, the company will remove the messages of users who opted to designate their account “protected,” so that only people who obtain their explicit permission can follow them.

Image Credit... Christophe Vorlet

A Twitter user can also elect to use a pseudonym and not share any personally identifying information. Twitter does not add identity tags that match its users to real people.

Each message is accompanied by some tidbits of supplemental information, like the number of followers that the author had at the time and how many users the author was following. While Mr. Cohen said it would be useful for a historian to know who the followers and the followed are, this information is not included in the Tweet itself.

But there’s nothing private about who follows whom among users of Twitter’s unprotected, public accounts. This information is displayed both at Twitter’s own site and in applications developed by third parties whom Twitter welcomes to tap its database.

Alexander Macgillivray, Twitter’s general counsel, said, “From the beginning, Twitter has been a public and open service.” Twitter’s privacy policy states: “Our services are primarily designed to help you share information with the world. Most of the information you provide to us is information you are asking us to make public.”

Mr. Macgillivray added, “That’s why, when we were revising our privacy policy, we toyed with the idea of calling it our ‘public policy.’ ” He said the company would have done so but California law required that it have a “privacy policy” labeled as such.