Q: When you say “click on somebody’s profile,” you mean you save our viewing history?

Facebook Employee: That’s right. How do you think we know who your best friends are?

Johnson has always had a geek’s penchant for self-education, and in that spirit he cultivated a side interest, and ultimately an expertise, in writing computer code. His Web log, which he named “Little Green Footballs” (a private joke whose derivation he has always refused to divulge), was begun in February 2001 mostly as a way to share advice and information with fellow code jockeys — his approach was similar in outlook, if vastly larger in its reach, to the guiding spirit in the days of ham radio. His final post on Sept. 10, 2001, was titled “Placement of Web Page Elements.” It read, in its entirety: “Here’s a well-executed academic study of where users expect things to be on a typical Web page.” It linked to, well, exactly what it said. The post attracted one comment, which read, in its entirety, “Fantastic article.”