Email, apparently, is now “old school.”

The number of total unique visitors in the United States to major e-mail sites like Yahoo and Hotmail is now in steady decline, according to the research company comScore. Such visits peaked in November 2009 and have since slid 6 percent; visits among 12- to 17-year-olds fell around 18 percent. (The only big gainer in the category has been Gmail, up 10 percent from a year ago.)

The slide in e-mail does not reflect a drop in digital communication; people have just gravitated to instant messaging, texting and Facebook (four billion messages daily).

That Facebook is the new new should come as no surprise.

Lena Jenny, 17, a high school senior in Cupertino, Calif., said texting was so quick that “I sometimes have an answer before I even shut my phone.” E-mail, she added, is “so lame.”

Facebook is trying to appeal to the Lenas of the world. It is rolling out a revamped messaging service that is intended to feel less like e-mail and more like texting.

The company decided to eliminate the subject line on messages after its research showed that it was most commonly left blank or used for an uninformative “hi” or “yo.”

Facebook also killed the “cc” and “bcc” lines. And hitting the enter key can immediately fire off the message, à la instant messaging, instead of creating a new paragraph. The changes, company executives say, leave behind time-consuming formalities that separate users from what they crave: instant conversation.

Facebook, of course, has a horrifying record regarding security and privacy. The site is literally an Openbook. So what will happen once it owns half a billion people’s private messages? If nothing else, it’s going to be funny.