Facebook hits a billion users, but what does that mean?

Media Outlet: 

E-Commerce Times 

Facebook recently announced it reached 1 billion active monthly users. Steven L. Johnson, assistant professor of management information systems at Temple's Fox School of Business, said the number doesn't say anything about how people are using Facebook. “Some are on Facebook all day long, but others may have visited the site only once to create an account. What brands and advertisers care about is user engagement, not just the number of users."