If you haven't joined Facebook already, you should do so because... everyone else is doing it.
Seriously though, it's so popular that most people you know are already on it. I'd like the standard to be neutral like email too, but it's not; it's Facebook. And if you want to be where your friends and loved ones are, here it is.
Want to send me an email? I'll get it, eventually. 90% of the email I get is of little interest to me, so I check it a couple of times a week. Every message I see on Facebook is from someone I told them is a friend of mine. I check it every day.
Oh, and do you have my email address? If you don't, you need to get that to send me an email. With Facebook you can search for someone's real name. But you rarely need to. Most people I know are already friends of other friends. Once you connect with one, it's easy to connect to the rest.
Facebook messaging can be used like email, with long full length messages. Or you can use it for brief back and fourth messages. It keeps a feed of each conversation partner, or partners if you're having a group conversation. And speaking of group conversations, it's a much easier way to make sure your participants are not crossing paths, and responding to older messages.
But the biggest reason to join is that people you care about are living their lives there. People you don't see everyday will post updates. They'll comment on other people's updates. Thoughts, opinions, and events are happening that you wouldn't know about otherwise.