In some cases that might be true, but I'm observing a lot of communication among people my age (mid/late twenties) that used to happen over email is moving to Facebook. I'm Facebook friends with a number of people that I spend time with regularly, but I don't have their email addresses.

Here's why: it's harder to give someone your email address than to become Facebook friends with them. For email, you have to write down a long string, keep track of it until you get home, and correctly type it into a computer. For Facebook, you just say, "Are you on Facebook? Are you friends with Jon? Just look me up on his friends list."

Facebook is a resource for contact information. Most people put it on their profile.

Inside of Facebook, I think that's true. Despite the superficial similarities with e-mail, I think Facebook messages are more like long-form tweets.