After all of the speculation over the past week about a new Facebook e-mail client, the company today announced a more broadly defined “Modern Messaging System.”

Explicitly positioned by CEO Mark Zuckerberg as “not an e-mail killer,” this will include three main components: seamless messaging, conversation history and a social inbox. The general idea is that messages aren’t put into traditional format dilos but rather come across in a variety of ways that are most appropriate for the device you’re on at any given time.

This makes sense as Facebook has about 200 million active mobile users (out of a total of half a billion users globally). The other driver here was the notion that e-mail for some is too formal. Shorter message formats like SMS and IM are more appropriate for lots of the messaging that Facebook currently sees.

As for the actual integration, this messaging will all be accessed and organized in the new “social inbox.” Users will be offered @facebook e-mail addresses but the mail client can also be set up with your existing e-mail address from separate services like GMail.

There are lots of implications for this, many of which we covered in the previous post. Most notably this makes Facebook more “sticky” and will boost page views and session lengths. And it clearly thickens the glue that holds together its various social assets and functional areas such as photos and events.

This could develop in other ways and in new formats such as voice. But for now, it’s all about text. More to come on this as it rolls out.