SAN FRANCISCO — Facebook is betting that one day soon, we'll all be acting like high school students — more texting and instant-messaging, at the expense of e-mail.Facebook unveiled a new messaging system Monday, and while CEO Mark Zuckerberg didn't go as far as declaring e-mail dead, he clearly sees the four-decade-old technology being eclipsed by more real-time ways of communicating."We don't think a modern messaging system is going to be e-mail," Zuckerberg said.Right now, Facebook's Messages section is a lot like an e-mail inbox. The overhauled version, which will be rolled out to users by invitation in coming months, brings in cell phone texts, IM chats and e-mails from non-Facebook accounts.