Hi George,
The thing with Facebook is, if one of your friends is online, there is a chat facility that is private between you and your friend, so messaging is pretty instant and I can imagine some would rather use this function than a conventional email. There is also a messaging function which works pretty much the same as an email whereby you have an inbox. As for photo's, you also have options as to who can see it such as friends only.
You can also delete people off your friends list if things start to go wrong or nasty.
As for Facebook having a focus, well, it doesn't really, it's as you say, a means to catch up and chat with friends in pretty much the same way as you would if you met them in person!
Ray